Initial commit: Books accounting system with EventFlow CQRS

Backend (.NET 10): - EventFlow CQRS/Event Sourcing with PostgreSQL - GraphQL.NET API with mutations and queries - Custom ReadModelSqlGenerator for snake_case PostgreSQL columns - Hangfire for background job processing - Integration tests with isolated test databases Frontend (React/Vite): - Initial project structure 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
2026-01-18 02:52:30 +01:00 · 2026-01-18 02:52:30 +01:00 · 66f6fa138d
commit 66f6fa138d
126 changed files with 24741 additions and 0 deletions
--- a/.gitignore
+++ b/.gitignore
@ -0,0 +1,204 @@
+# ============================================
+# .NET / C#
+# ============================================
+[Bb]in/
+[Oo]bj/
+[Oo]ut/
+[Ll]og/
+[Ll]ogs/
+*.user
+*.rsuser
+*.suo
+*.userosscache
+*.sln.docstates
+*.userprefs
+*.pidb
+*.booproj
+*.svd
+*.pdb
+*.idb
+*.ilk
+*.meta
+*.obj
+*.pch
+*.pdb
+*.pgc
+*.pgd
+*.rsp
+*.sbr
+*.tlb
+*.tli
+*.tlh
+*.tmp
+*.vspscc
+*.vssscc
+.builds
+*.DotSettings.user
+
+# NuGet
+*.nupkg
+*.snupkg
+**/[Pp]ackages/*
+!**/[Pp]ackages/build/
+.nuget/
+project.lock.json
+project.fragment.lock.json
+artifacts/
+
+# .NET Core
+*.PublishPersistentProperties.json
+
+# ============================================
+# Node.js / Frontend
+# ============================================
+node_modules/
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+.pnpm-store/
+.npm/
+.yarn/
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions
+
+# Vite
+.vite/
+dist/
+dist-ssr/
+*.local
+
+# Build outputs
+build/
+.next/
+.nuxt/
+.output/
+.cache/
+
+# ============================================
+# Docker
+# ============================================
+**/docker-compose.override.yml
+**/.docker/
+
+# ============================================
+# Kubernetes
+# ============================================
+*.kubeconfig
+kubeconfig
+**/charts/*.tgz
+**/*.decrypted.yaml
+**/secrets.yaml
+**/secrets.yml
+!**/secrets.example.yaml
+
+# Helm
+**/Chart.lock
+
+# ============================================
+# IDEs and Editors
+# ============================================
+
+# JetBrains Rider / IntelliJ
+.idea/
+*.sln.iml
+
+# Visual Studio Code
+.vscode/*
+!.vscode/settings.json
+!.vscode/tasks.json
+!.vscode/launch.json
+!.vscode/extensions.json
+!.vscode/*.code-snippets
+.history/
+*.vsix
+
+# Visual Studio
+.vs/
+*.VC.db
+*.VC.VC.opendb
+
+# ============================================
+# OS Generated
+# ============================================
+
+# macOS
+.DS_Store
+.AppleDouble
+.LSOverride
+._*
+.DocumentRevisions-V100
+.fseventsd
+.Spotlight-V100
+.TemporaryItems
+.Trashes
+.VolumeIcon.icns
+.com.apple.timemachine.donotpresent
+.AppleDB
+.AppleDesktop
+Network Trash Folder
+Temporary Items
+.apdisk
+
+# Windows
+Thumbs.db
+Thumbs.db:encryptable
+ehthumbs.db
+ehthumbs_vista.db
+*.stackdump
+[Dd]esktop.ini
+$RECYCLE.BIN/
+*.cab
+*.msi
+*.msix
+*.msm
+*.msp
+*.lnk
+
+# Linux
+*~
+.fuse_hidden*
+.directory
+.Trash-*
+.nfs*
+
+# ============================================
+# Environment & Secrets
+# ============================================
+.env
+.env.*
+!.env.example
+!.env.*.example
+*.pem
+*.key
+*.p12
+*.pfx
+appsettings.Development.json
+appsettings.Local.json
+secrets.json
+
+# ============================================
+# Testing
+# ============================================
+coverage/
+*.lcov
+.nyc_output/
+TestResults/
+*.trx
+
+# ============================================
+# Misc
+# ============================================
+*.log
+*.tmp
+*.temp
+*.swp
+*.swo
+*.bak
+*.backup
+.sass-cache/
+*.css.map
+*.js.map
--- a/README.md
+++ b/README.md
@ -0,0 +1,78 @@
+# Books - Dansk Bogføringssystem
+
+Et komplet bogføringssystem til danske virksomheder med support for regnskabsår, momsindberetning og SKAT-compliance.
+
+## Projektstruktur
+
+```
+books/
+├── frontend/          # React/TypeScript frontend
+│   ├── src/
+│   │   ├── components/
+│   │   ├── pages/
+│   │   ├── stores/
+│   │   ├── hooks/
+│   │   ├── lib/
+│   │   └── types/
+│   ├── package.json
+│   └── vite.config.ts
+│
+├── backend/           # .NET 8 Web API
+│   ├── Books.Api/
+│   │   ├── Controllers/
+│   │   ├── Models/
+│   │   ├── Services/
+│   │   └── Program.cs
+│   ├── Books.slnx
+│   └── BACKEND_REQUIREMENTS.md
+│
+└── README.md
+```
+
+## Teknologi Stack
+
+### Frontend
+- React 18 med TypeScript
+- Ant Design komponenter
+- Zustand state management
+- Vite build tool
+- Day.js til dato-håndtering
+
+### Backend
+- .NET 10 Web API
+- C# 14
+- Entity Framework Core
+- PostgreSQL database
+- JWT authentication
+- SLNX solution format
+
+## Kom i gang
+
+### Frontend
+```bash
+cd frontend
+npm install
+npm run dev
+```
+
+### Backend
+```bash
+cd backend
+dotnet restore
+dotnet run --project Books.Api
+```
+
+## Features
+
+- [x] Regnskabsår (Fiscal Years) - opret, luk, lås
+- [x] Regnskabsperioder - månedlig, kvartalsvis, halvårlig, årlig
+- [x] Kontoplan med danske standardkonti
+- [x] Hurtig bogføring interface
+- [ ] Årsafslutning med lukkeposter
+- [ ] Dynamiske åbningsbalancer
+- [ ] Momsindberetning
+- [ ] SKAT integration
+
+## Licens
+
+Proprietary - Alle rettigheder forbeholdes.
--- a/account-suggestions/accounts-with-suggestions.json
+++ b/account-suggestions/accounts-with-suggestions.json
--- a/account-suggestions/accounts.json
+++ b/account-suggestions/accounts.json
--- a/account-suggestions/accounts.toon
+++ b/account-suggestions/accounts.toon
@ -0,0 +1,122 @@
+# Dinero Expense Accounts with Suggestions
+# 104 accounts for AI-powered account suggestion
+
+meta:
+  source: Dinero API
+  organizationId: 582288
+  accountType: expense
+  totalAccounts: 104
+  removedAccounts: 3220|5120|7140|7280
+
+regionVariants:
+  varekøb: DK=2000|EU=2050|WORLD=2150
+  ydelseskøb: EU=2100|WORLD=2200
+  fragt: DK=2250|DK_VATFREE=2300|EU=2350
+  husleje: DK=5000|DK_VATFREE=5010
+  parkering: DK=6080|DK_VATFREE=6085
+
+accounts[104]{number,name,category,vatCode,region,vatRubric,suggestions}:
+  2000,Vareforbrug,Variable omkostninger,I25,DK,,varer|indkøb|lager|goods|inventory|purchase|varekøb
+  2050,EU-erhvervelser varer (rubrik A-varer),Variable omkostninger,IEUV,EU,EU_BOX_A_GOODS,eu|europa|germany|france|italien|spain|netherlands|eu-erhvervelse
+  2100,EU-erhvervelser ydelser (rubrik A-ydelser),Variable omkostninger,IEUY,EU,EU_BOX_A_SERVICES,eu|europa|ydelse|service|eu-ydelse
+  2150,Varekøb verden,Variable omkostninger,IVV,WORLD,,usa|uk|kina|china|verden|import|world
+  2200,Ydelseskøb verden,Variable omkostninger,IVY,WORLD,,usa|uk|kina|verden|service|ydelse|world
+  2250,Fragt med moms,Variable omkostninger,I25,DK,,fragt|levering|shipping|delivery|postnord|gls|dao|bring|ups|dhl
+  2300,Fragt uden moms,Variable omkostninger,,DK,,fragt|shipping|delivery|momsfri
+  2350,Fragt - EU,Variable omkostninger,IEUV,EU,EU_BOX_A_GOODS,fragt|eu|shipping|dhl|ups|fedex
+  2400,Valutakursdifferencer import,Variable omkostninger,,DK,,valuta|kurs|currency|exchange
+  2450,Varelagerregulering,Variable omkostninger,,DK,,lager|regulering|inventory
+  2800,Fremmed arbejde,Variable omkostninger,I25,DK,,underleverandør|freelance|contractor|subcontractor|fremmed arbejde
+  3000,AM-indkomst,Lønomkostninger,,DK,,løn|salary|wage|payroll|am-indkomst
+  3020,Arbejdsgiver ATP,Lønomkostninger,,DK,,atp|arbejdsgiver
+  3040,Medarbejder ATP,Lønomkostninger,,DK,,atp|medarbejder
+  3060,Sygepenge mv.,Lønomkostninger,,DK,,sygepenge|sickness|sygedag
+  3070,Personalegoder herunder fri telefon,Lønomkostninger,,DK,,personalegode|fri telefon|benefit|goder
+  3080,B-honorar,Lønomkostninger,,DK,,b-honorar|honorar|fee
+  3090,Barsel,Lønomkostninger,,DK,,barsel|maternity|fødsel
+  3100,Feriepenge og SH,Lønomkostninger,,DK,,ferie|feriepenge|sh|vacation|holiday
+  3120,Pension,Lønomkostninger,,DK,,pension|pensionsbidrag|retirement
+  3140,Diæter/rejsegodtgørelse,Lønomkostninger,,DK,,diæt|rejsegodtgørelse|per diem|allowance
+  3160,Kørsel i egen bil (kilometergodtgørelse),Lønomkostninger,,DK,,kørsel|kilometer|km|mileage|km-godtgørelse|bil
+  3180,AER/AES/ATP-finansieringsbidrag,Lønomkostninger,,DK,,aer|aes|atp|finansieringsbidrag
+  3200,Arbejdstøj,Lønomkostninger,I25,DK,,arbejdstøj|uniform|workwear|tøj
+  3240,Mad under kursus/møder mv. fuldt fradrag,Lønomkostninger,I25,DK,,mad|kursus|møde|food|catering|frokost
+  3260,Gaver til personalet fuldt fradrag,Lønomkostninger,,DK,,gave|personale|gift|employee
+  3280,Uddannelsesudgifter,Lønomkostninger,I25,DK,,kursus|uddannelse|course|training|efteruddannelse
+  3300,Diverse vedr. ansatte med moms,Lønomkostninger,I25,DK,,diverse|personale|ansatte|moms
+  3320,Diverse vedr. ansatte uden moms,Lønomkostninger,,DK,,diverse|personale|ansatte|momsfri
+  3340,Regulering feriepenge,Lønomkostninger,,DK,,ferie|regulering|feriepenge
+  3355,Frokostordning til ansatte,Lønomkostninger,,DK,,frokost|frokostordning|lunch|kantine
+  4000,Annoncer og reklame,Salgsomkostninger,I25,DK,,annonce|reklame|marketing|ads|google|facebook|linkedin|meta|advertising
+  4020,Udsmykning i forbindelse med arrangementer/events,Salgsomkostninger,I25,DK,,udsmykning|event|arrangement|decoration
+  4040,Hotel personale fuldt fradrag,Salgsomkostninger,I25,DK,,hotel|overnatning|accommodation|booking.com|hotels.com|personale
+  4060,Hotel forretningsforbindelser delvis fradrag,Salgsomkostninger,I25,DK,,hotel|overnatning|kunde|client|forretningsforbindelse
+  4080,Konferencer,Salgsomkostninger,,DK,,konference|conference|seminar|event
+  4100,Messer,Salgsomkostninger,I25,DK,,messe|fair|exhibition|udstilling
+  4120,Restaurant personale fuldt fradrag,Salgsomkostninger,REP,DK,,restaurant|frokost|middag|lunch|dinner|café|personale
+  4140,Restaurant forretningsforbindelser delvis fradrag,Salgsomkostninger,REP,DK,,restaurant|repræsentation|kunde|client|forretningsforbindelse
+  4160,Mad i virksomheden til forretningsforbindelser delvis fradrag,Salgsomkostninger,I25,DK,,mad|virksomhed|forretningsforbindelse|catering
+  4180,Repræsentation gaver og blomster delvis fradrag,Salgsomkostninger,,DK,,gave|blomster|gift|flowers|repræsentation
+  4200,Anden fradragsberettiget repræsentation med moms,Salgsomkostninger,I25,DK,,repræsentation|representation
+  4220,Repræsentation diverse,Salgsomkostninger,,DK,,repræsentation|diverse
+  4240,Øvrige personaleomkostninger,Salgsomkostninger,I25,DK,,personale|personnel|staff
+  4260,Ej fradragsberettiget andel,Salgsomkostninger,,DK,,ikke-fradrag|non-deductible
+  4280,Rejseomkostninger,Salgsomkostninger,,DK,,rejse|travel|tur|trip
+  5000,Husleje,Lokaleomkostninger,I25,DK,,husleje|leje|rent|kontor|lokale
+  5010,Husleje uden moms,Lokaleomkostninger,,DK,,husleje|leje|rent|momsfri
+  5025,El,Lokaleomkostninger,I25,DK,,el|strøm|electricity|power|ørsted|norlys|ewii|andel
+  5030,Vand,Lokaleomkostninger,I25,DK,,vand|water|vandværk
+  5035,Varme,Lokaleomkostninger,I25,DK,,varme|gas|fjernvarme|heating|naturgas
+  5040,Elafgift,Lokaleomkostninger,,DK,,elafgift|afgift|tax
+  5045,Naturgas- og bygasafgift,Lokaleomkostninger,,DK,,gas|afgift|naturgas
+  5050,Vandafgift,Lokaleomkostninger,,DK,,vand|afgift|vandafgift
+  5060,Rengøring og affaldshåndtering,Lokaleomkostninger,I25,DK,,rengøring|cleaning|affald|waste|renovation
+  5080,Reparation og vedligeholdelse,Lokaleomkostninger,I25,DK,,reparation|vedligeholdelse|maintenance|repair
+  5100,Ejendomsskat,Lokaleomkostninger,,DK,,ejendomsskat|property tax|skat
+  5140,Mødelokaler,Lokaleomkostninger,I25,DK,,mødelokale|meeting room|konference
+  5160,Dekoration,Lokaleomkostninger,I25,DK,,dekoration|decoration|indretning
+  6000,Billeje (gulplade),Kørsel og rejser,I25,DK,,billeje|car rental|leje|hertz|avis|europcar|sixt|enterprise
+  6020,Brændstof (gulplade),Kørsel og rejser,I25,DK,,benzin|diesel|fuel|brændstof|ok|shell|q8|circle k|ingo
+  6040,Vedligeholdelse af bil (gulplade),Kørsel og rejser,I25,DK,,bil|vedligeholdelse|car|service|værksted|dæk
+  6060,Vægtafgift og forsikringer,Kørsel og rejser,,DK,,vægtafgift|forsikring|car insurance|bilforsikring
+  6080,Parkering (gulplade),Kørsel og rejser,I25,DK,,parkering|parking|p-afgift|easypark|parkman|apcoa
+  6085,Parkering uden moms,Kørsel og rejser,,DK,,parkering|parking|momsfri
+  6100,Broafgift,Kørsel og rejser,I25,DK,,bro|bridge|storebælt|øresund|toll|afgift
+  6120,Taxa,Kørsel og rejser,,DK,,taxa|taxi|uber|bolt|dantaxi|viggo|4x27
+  6140,Tog,Kørsel og rejser,,DK,,tog|dsb|train|rejsekort|billet
+  6160,Fly,Kørsel og rejser,,DK,,fly|flight|sas|norwegian|ryanair|lufthansa|flybillet
+  6180,Bus,Kørsel og rejser,,DK,,bus|movia|arriva|flixbus
+  6200,Færge,Kørsel og rejser,I25,DK,,færge|ferry|molslinjen|scandlines|bornholmslinjen
+  6400,Diverse transportomkostninger uden moms,Kørsel og rejser,,DK,,transport|diverse|transportation
+  7005,Revision og regnskabsmæssig assistance,Administrationsomkostninger,I25,DK,,revisor|revision|audit|regnskab|pwc|deloitte|ey|kpmg|bdo
+  7010,Advokat,Administrationsomkostninger,I25,DK,,advokat|lawyer|legal|juridisk|attorney
+  7020,Bogføringsassistance,Administrationsomkostninger,I25,DK,,bogføring|bookkeeping|regnskab|bogholder
+  7040,Konsulentbistand,Administrationsomkostninger,I25,DK,,konsulent|consultant|rådgivning|advisory|rådgiver
+  7060,Kontingenter inkl. moms,Administrationsomkostninger,I25,DK,,kontingent|medlemskab|membership|forening
+  7080,Kontingenter ekskl. moms,Administrationsomkostninger,,DK,,kontingent|medlemskab|momsfri
+  7100,Aviser,Administrationsomkostninger,,DK,,avis|newspaper|berlingske|politiken|jyllandsposten|børsen|dagblad
+  7120,Faglitteratur,Administrationsomkostninger,I25,DK,,bog|book|litteratur|amazon|saxo|tales|faglitteratur
+  7160,Erhvervsforsikringer,Administrationsomkostninger,,DK,,forsikring|insurance|tryg|topdanmark|if|codan|gjensidige|alm brand
+  7180,Fragt og kørsel,Administrationsomkostninger,I25,DK,,fragt|kørsel|shipping|transport
+  7200,Kontorartikler og tryksager,Administrationsomkostninger,I25,DK,,kontor|office|papir|printer|lyreco|staples|kontorartikler
+  7220,Porto og gebyrer,Administrationsomkostninger,,DK,,porto|gebyr|postage|fee|postnord
+  7240,Telefoni,Administrationsomkostninger,I25,DK,,telefon|mobil|phone|mobile|telia|telenor|3|telmore|cbs|lebara
+  7260,Beskatning af fri telefoni,Administrationsomkostninger,,DK,,telefon|beskatning|fri telefon
+  7300,Internet og webhotel,Administrationsomkostninger,I25,DK,,internet|hosting|domain|web|simply|one.com|cloudflare|aws|azure
+  7320,Køb af software,Administrationsomkostninger,I25,DK,,software|app|abonnement|subscription|saas|adobe|microsoft|slack|notion|google workspace|dropbox
+  7360,Offentlige bøder og gebyrer,Administrationsomkostninger,,DK,,bøde|gebyr|fine|offentlig
+  7380,Registrerede kassedifferencer,Administrationsomkostninger,,DK,,kasse|difference|cash
+  7400,Betalingsløsning,Administrationsomkostninger,I25,DK,,betaling|payment|stripe|mobilepay|nets|clearhaus|paypal
+  7420,Indløsere,Administrationsomkostninger,,DK,,indløser|acquirer|nets|clearhaus
+  7440,Licens,Administrationsomkostninger,,DK,,licens|license|rettighed
+  7460,Diverse inkl. moms,Administrationsomkostninger,I25,DK,,diverse|moms|miscellaneous
+  7480,Diverse ekskl. moms,Administrationsomkostninger,,DK,,diverse|momsfri|miscellaneous
+  7500,Generalforsamling bestyrelsesmøder ude i byen fuld fradrag,Administrationsomkostninger,REP,DK,,generalforsamling|bestyrelse|møde|board|meeting
+  7520,Generalforsamling i virksomhedens lokaler,Administrationsomkostninger,,DK,,generalforsamling|møde|lokale
+  7540,Bestyrelsesmøder i virksomhedens lokaler,Administrationsomkostninger,I25,DK,,bestyrelse|board|møde|lokale
+  7560,Bestyrelsesmøder ude i byen,Administrationsomkostninger,REP,DK,,bestyrelse|board|møde|restaurant
+  8040,Småanskaffelser (straksafskrivning),Afskrivninger,I25,DK,,computer|laptop|møbler|furniture|udstyr|equipment|apple|dell|lenovo|småanskaffelse
+  8050,Småanskaffelser med omvendt betalingspligt,Afskrivninger,OBPK,DK,,småanskaffelse|omvendt betalingspligt|reverse charge
+  9200,Bankrenter,Renteudgifter,,DK,,rente|bank|interest|bankrenter
+  9210,Leverandører mv.,Renteudgifter,,DK,,rente|leverandør|supplier|interest
+  9220,Ikke-fradragsberettigede renter,Renteudgifter,,DK,,rente|ikke-fradrag|non-deductible
--- a/account-suggestions/fetch-accounts.sh
+++ b/account-suggestions/fetch-accounts.sh
--- a/account-suggestions/process-accounts.js
+++ b/account-suggestions/process-accounts.js
@ -0,0 +1,199 @@
+const fs = require('fs');
+
+// Load accounts
+const accounts = JSON.parse(fs.readFileSync('accounts.json', 'utf8'));
+
+// Accounts to remove
+const removeNumbers = [3220, 5120, 7140, 7280];
+
+// Suggestions mapping by account number
+const suggestionsMap = {
+  // Variable omkostninger
+  2000: ["varer", "indkøb", "lager", "goods", "inventory", "purchase", "varekøb"],
+  2050: ["eu", "europa", "germany", "france", "italien", "spain", "netherlands", "eu-erhvervelse"],
+  2100: ["eu", "europa", "ydelse", "service", "eu-ydelse"],
+  2150: ["usa", "uk", "kina", "china", "verden", "import", "world"],
+  2200: ["usa", "uk", "kina", "verden", "service", "ydelse", "world"],
+  2250: ["fragt", "levering", "shipping", "delivery", "postnord", "gls", "dao", "bring", "ups", "dhl"],
+  2300: ["fragt", "shipping", "delivery", "momsfri"],
+  2350: ["fragt", "eu", "shipping", "dhl", "ups", "fedex"],
+  2400: ["valuta", "kurs", "currency", "exchange"],
+  2450: ["lager", "regulering", "inventory"],
+  2800: ["underleverandør", "freelance", "contractor", "subcontractor", "fremmed arbejde"],
+
+  // Lønomkostninger
+  3000: ["løn", "salary", "wage", "payroll", "am-indkomst"],
+  3020: ["atp", "arbejdsgiver"],
+  3040: ["atp", "medarbejder"],
+  3060: ["sygepenge", "sickness", "sygedag"],
+  3070: ["personalegode", "fri telefon", "benefit", "goder"],
+  3080: ["b-honorar", "honorar", "fee"],
+  3090: ["barsel", "maternity", "fødsel"],
+  3100: ["ferie", "feriepenge", "sh", "vacation", "holiday"],
+  3120: ["pension", "pensionsbidrag", "retirement"],
+  3140: ["diæt", "rejsegodtgørelse", "per diem", "allowance"],
+  3160: ["kørsel", "kilometer", "km", "mileage", "km-godtgørelse", "bil"],
+  3180: ["aer", "aes", "atp", "finansieringsbidrag"],
+  3200: ["arbejdstøj", "uniform", "workwear", "tøj"],
+  3240: ["mad", "kursus", "møde", "food", "catering", "frokost"],
+  3260: ["gave", "personale", "gift", "employee"],
+  3280: ["kursus", "uddannelse", "course", "training", "efteruddannelse"],
+  3300: ["diverse", "personale", "ansatte", "moms"],
+  3320: ["diverse", "personale", "ansatte", "momsfri"],
+  3340: ["ferie", "regulering", "feriepenge"],
+  3355: ["frokost", "frokostordning", "lunch", "kantine"],
+
+  // Salgsomkostninger
+  4000: ["annonce", "reklame", "marketing", "ads", "google", "facebook", "linkedin", "meta", "advertising"],
+  4020: ["udsmykning", "event", "arrangement", "decoration"],
+  4040: ["hotel", "overnatning", "accommodation", "booking.com", "hotels.com", "personale"],
+  4060: ["hotel", "overnatning", "kunde", "client", "forretningsforbindelse"],
+  4080: ["konference", "conference", "seminar", "event"],
+  4100: ["messe", "fair", "exhibition", "udstilling"],
+  4120: ["restaurant", "frokost", "middag", "lunch", "dinner", "café", "personale"],
+  4140: ["restaurant", "repræsentation", "kunde", "client", "forretningsforbindelse"],
+  4160: ["mad", "virksomhed", "forretningsforbindelse", "catering"],
+  4180: ["gave", "blomster", "gift", "flowers", "repræsentation"],
+  4200: ["repræsentation", "representation"],
+  4220: ["repræsentation", "diverse"],
+  4240: ["personale", "personnel", "staff"],
+  4260: ["ikke-fradrag", "non-deductible"],
+  4280: ["rejse", "travel", "tur", "trip"],
+
+  // Lokaleomkostninger
+  5000: ["husleje", "leje", "rent", "kontor", "lokale"],
+  5010: ["husleje", "leje", "rent", "momsfri"],
+  5025: ["el", "strøm", "electricity", "power", "ørsted", "norlys", "ewii", "andel"],
+  5030: ["vand", "water", "vandværk"],
+  5035: ["varme", "gas", "fjernvarme", "heating", "naturgas"],
+  5040: ["elafgift", "afgift", "tax"],
+  5045: ["gas", "afgift", "naturgas"],
+  5050: ["vand", "afgift", "vandafgift"],
+  5060: ["rengøring", "cleaning", "affald", "waste", "renovation"],
+  5080: ["reparation", "vedligeholdelse", "maintenance", "repair"],
+  5100: ["ejendomsskat", "property tax", "skat"],
+  5140: ["mødelokale", "meeting room", "konference"],
+  5160: ["dekoration", "decoration", "indretning"],
+
+  // Kørsel og rejser
+  6000: ["billeje", "car rental", "leje", "hertz", "avis", "europcar", "sixt", "enterprise"],
+  6020: ["benzin", "diesel", "fuel", "brændstof", "ok", "shell", "q8", "circle k", "ingo"],
+  6040: ["bil", "vedligeholdelse", "car", "service", "værksted", "dæk"],
+  6060: ["vægtafgift", "forsikring", "car insurance", "bilforsikring"],
+  6080: ["parkering", "parking", "p-afgift", "easypark", "parkman", "apcoa"],
+  6085: ["parkering", "parking", "momsfri"],
+  6100: ["bro", "bridge", "storebælt", "øresund", "toll", "afgift"],
+  6120: ["taxa", "taxi", "uber", "bolt", "dantaxi", "viggo", "4x27"],
+  6140: ["tog", "dsb", "train", "rejsekort", "billet"],
+  6160: ["fly", "flight", "sas", "norwegian", "ryanair", "lufthansa", "flybillet"],
+  6180: ["bus", "movia", "arriva", "flixbus"],
+  6200: ["færge", "ferry", "molslinjen", "scandlines", "bornholmslinjen"],
+  6400: ["transport", "diverse", "transportation"],
+
+  // Administrationsomkostninger
+  7005: ["revisor", "revision", "audit", "regnskab", "pwc", "deloitte", "ey", "kpmg", "bdo"],
+  7010: ["advokat", "lawyer", "legal", "juridisk", "attorney"],
+  7020: ["bogføring", "bookkeeping", "regnskab", "bogholder"],
+  7040: ["konsulent", "consultant", "rådgivning", "advisory", "rådgiver"],
+  7060: ["kontingent", "medlemskab", "membership", "forening"],
+  7080: ["kontingent", "medlemskab", "momsfri"],
+  7100: ["avis", "newspaper", "berlingske", "politiken", "jyllandsposten", "børsen", "dagblad"],
+  7120: ["bog", "book", "litteratur", "amazon", "saxo", "tales", "faglitteratur"],
+  7160: ["forsikring", "insurance", "tryg", "topdanmark", "if", "codan", "gjensidige", "alm brand"],
+  7180: ["fragt", "kørsel", "shipping", "transport"],
+  7200: ["kontor", "office", "papir", "printer", "lyreco", "staples", "kontorartikler"],
+  7220: ["porto", "gebyr", "postage", "fee", "postnord"],
+  7240: ["telefon", "mobil", "phone", "mobile", "telia", "telenor", "3", "telmore", "cbs", "lebara"],
+  7260: ["telefon", "beskatning", "fri telefon"],
+  7300: ["internet", "hosting", "domain", "web", "simply", "one.com", "cloudflare", "aws", "azure"],
+  7320: ["software", "app", "abonnement", "subscription", "saas", "adobe", "microsoft", "slack", "notion", "google workspace", "dropbox"],
+  7360: ["bøde", "gebyr", "fine", "offentlig"],
+  7380: ["kasse", "difference", "cash"],
+  7400: ["betaling", "payment", "stripe", "mobilepay", "nets", "clearhaus", "paypal"],
+  7420: ["indløser", "acquirer", "nets", "clearhaus"],
+  7440: ["licens", "license", "rettighed"],
+  7460: ["diverse", "moms", "miscellaneous"],
+  7480: ["diverse", "momsfri", "miscellaneous"],
+  7500: ["generalforsamling", "bestyrelse", "møde", "board", "meeting"],
+  7520: ["generalforsamling", "møde", "lokale"],
+  7540: ["bestyrelse", "board", "møde", "lokale"],
+  7560: ["bestyrelse", "board", "møde", "restaurant"],
+
+  // Afskrivninger
+  8040: ["computer", "laptop", "møbler", "furniture", "udstyr", "equipment", "apple", "dell", "lenovo", "småanskaffelse"],
+  8050: ["småanskaffelse", "omvendt betalingspligt", "reverse charge"],
+
+  // Renteudgifter
+  9200: ["rente", "bank", "interest", "bankrenter"],
+  9210: ["rente", "leverandør", "supplier", "interest"],
+  9220: ["rente", "ikke-fradrag", "non-deductible"]
+};
+
+// VAT rubric mapping for EU accounts
+const vatRubricMap = {
+  2050: "EU_BOX_A_GOODS",
+  2100: "EU_BOX_A_SERVICES",
+  2350: "EU_BOX_A_GOODS"
+};
+
+// Region variants mapping
+const regionVariants = {
+  2000: { category: "varekøb", EU: 2050, WORLD: 2150 },
+  2050: { category: "varekøb", DK: 2000, WORLD: 2150 },
+  2150: { category: "varekøb", DK: 2000, EU: 2050 },
+  2100: { category: "ydelseskøb", WORLD: 2200 },
+  2200: { category: "ydelseskøb", EU: 2100 },
+  2250: { category: "fragt", DK_VATFREE: 2300, EU: 2350 },
+  2300: { category: "fragt", DK: 2250, EU: 2350 },
+  2350: { category: "fragt", DK: 2250, DK_VATFREE: 2300 },
+  5000: { category: "husleje", DK_VATFREE: 5010 },
+  5010: { category: "husleje", DK: 5000 },
+  6080: { category: "parkering", DK_VATFREE: 6085 },
+  6085: { category: "parkering", DK: 6080 }
+};
+
+// Process accounts
+const enrichedAccounts = accounts
+  .filter(acc => !removeNumbers.includes(acc.number))
+  .map(acc => {
+    // Determine region
+    let region = "DK";
+    if (acc.defaultVatTypeRegion === "EU") {
+      region = "EU";
+    } else if (acc.defaultVatTypeRegion === "World") {
+      region = "WORLD";
+    }
+
+    // Build enriched account
+    const enriched = {
+      id: acc.id,
+      number: acc.number,
+      name: acc.name,
+      categoryName: acc.categoryName,
+      defaultVatCode: acc.defaultVatCode,
+      region: region,
+      suggestions: suggestionsMap[acc.number] || []
+    };
+
+    // Add vatRubric for EU accounts
+    if (vatRubricMap[acc.number]) {
+      enriched.vatRubric = vatRubricMap[acc.number];
+    }
+
+    // Add related accounts for region variants
+    if (regionVariants[acc.number]) {
+      enriched.regionVariants = regionVariants[acc.number];
+    }
+
+    return enriched;
+  });
+
+// Save output
+fs.writeFileSync(
+  'accounts-with-suggestions.json',
+  JSON.stringify(enrichedAccounts, null, 2),
+  'utf8'
+);
+
+console.log(`Processed ${enrichedAccounts.length} accounts (removed ${removeNumbers.length})`);
+console.log('Saved to accounts-with-suggestions.json');
--- a/account-suggestions/test-invoices/Betalingspåmindelse_nr._2_fra_EWII.json
+++ b/account-suggestions/test-invoices/Betalingspåmindelse_nr._2_fra_EWII.json
@ -0,0 +1,45 @@
+{
+  "vendor": {
+    "name": "EWII A/S",
+    "address": "Kokbjerg 30, 6000 Kolding",
+    "country": "DK",
+    "vatNumber": "DK28297947"
+  },
+  "invoice": {
+    "type": "reminder",
+    "number": null,
+    "date": "2024-06-06",
+    "dueDate": "2024-06-17",
+    "currency": "DKK",
+    "totalAmount": 3361.04,
+    "vatAmount": null,
+    "netAmount": null
+  },
+  "customer": {
+    "name": "Nicolaj Helmer Hartmann",
+    "address": "Mejsevej 2, 8370 Hadsten",
+    "customerNumber": "400143507"
+  },
+  "lineItems": [
+    {
+      "description": "El restance - Rykker 2",
+      "amount": 3361.04
+    }
+  ],
+  "suggestedAccount": {
+    "number": 5025,
+    "name": "El",
+    "vatCode": "I25",
+    "region": "DK",
+    "confidence": 0.95,
+    "reasoning": "EWII er el-leverandør, betalingspåmindelse vedrører el-forbrug"
+  },
+  "alternativeAccounts": [
+    {
+      "number": 5040,
+      "name": "Elafgift",
+      "confidence": 0.3,
+      "reasoning": "Kan indeholde elafgift, men primært el-forbrug"
+    }
+  ]
+}
--- a/account-suggestions/test-invoices/Betalingspåmindelse_nr._2_fra_EWII.pdf
+++ b/account-suggestions/test-invoices/Betalingspåmindelse_nr._2_fra_EWII.pdf
--- a/account-suggestions/test-invoices/Invoice-44534E99-0004.pdf
+++ b/account-suggestions/test-invoices/Invoice-44534E99-0004.pdf
--- a/account-suggestions/test-invoices/Invoice-MVPAIAKP-0002.pdf
+++ b/account-suggestions/test-invoices/Invoice-MVPAIAKP-0002.pdf
--- a/account-suggestions/test-invoices/Microsoft
+++ b/account-suggestions/test-invoices/Microsoft
@ -0,0 +1,50 @@
+{
+  "vendor": {
+    "name": "Microsoft Ireland Operations Ltd",
+    "address": "One Microsoft Place, South County Business Park, Leopardstown, Dublin 18, D18 P521, Ireland",
+    "country": "IE",
+    "vatNumber": "IE8256796U"
+  },
+  "invoice": {
+    "type": "invoice",
+    "number": "E0800QGJD1",
+    "date": "2024-01-03",
+    "dueDate": "2024-01-03",
+    "currency": "DKK",
+    "totalAmount": 69.90,
+    "vatAmount": 0.00,
+    "netAmount": 69.90,
+    "vatNote": "omvendt betalingspligt"
+  },
+  "customer": {
+    "name": "Softwarehuset",
+    "address": "Mejsevej 2, 8370 Hadsten",
+    "vatNumber": "DK38059726"
+  },
+  "lineItems": [
+    {
+      "description": "Office 365 E1 - Månedligt abonnementsgebyr",
+      "period": "23-12-2023 - 22-01-2024",
+      "quantity": 1,
+      "unitPrice": 69.90,
+      "amount": 69.90
+    }
+  ],
+  "suggestedAccount": {
+    "number": 2100,
+    "name": "EU-erhvervelser ydelser (rubrik A-ydelser)",
+    "vatCode": "IEUY",
+    "region": "EU",
+    "vatRubric": "EU_BOX_A_SERVICES",
+    "confidence": 0.97,
+    "reasoning": "Software/cloud-service fra EU (Irland) med omvendt betalingspligt - skal bogføres som EU-ydelseskøb"
+  },
+  "alternativeAccounts": [
+    {
+      "number": 7320,
+      "name": "Køb af software",
+      "confidence": 0.4,
+      "reasoning": "Ville være korrekt hvis leverandør var dansk, men EU-leverandør kræver EU-konto pga. momsindberetning"
+    }
+  ]
+}
--- a/account-suggestions/test-invoices/Microsoft
+++ b/account-suggestions/test-invoices/Microsoft
--- a/account-suggestions/test-invoices/VACA_Mejlgade_2025-08-27_09_10_40.768_68aecb9075793e5d20cd70cc.pdf
+++ b/account-suggestions/test-invoices/VACA_Mejlgade_2025-08-27_09_10_40.768_68aecb9075793e5d20cd70cc.pdf
--- a/account-suggestions/test-invoices/Wolt+_2025-03-21_13_05_07.830_67dd64034768185bed932167.pdf
+++ b/account-suggestions/test-invoices/Wolt+_2025-03-21_13_05_07.830_67dd64034768185bed932167.pdf
--- a/account-suggestions/test-invoices/e90191aa-5f53-4aaf-bd70-298420938495.pdf
+++ b/account-suggestions/test-invoices/e90191aa-5f53-4aaf-bd70-298420938495.pdf
--- a/account-suggestions/test-invoices/fibia_receipt_72147924.json
+++ b/account-suggestions/test-invoices/fibia_receipt_72147924.json
@ -0,0 +1,65 @@
+{
+  "vendor": {
+    "name": "Waoo (Fibia P/S)",
+    "address": "Energivej 33, 4690 Haslev",
+    "country": "DK",
+    "vatNumber": "DK36058552"
+  },
+  "invoice": {
+    "type": "invoice",
+    "number": "08338955",
+    "date": "2025-08-07",
+    "dueDate": "2025-09-01",
+    "currency": "DKK",
+    "totalAmount": 1284.00,
+    "vatAmount": 256.80,
+    "netAmount": 1027.20
+  },
+  "customer": {
+    "name": "Nicolaj Hartmann",
+    "address": "Mejsevej 2, Hadbjerg, 8370 Hadsten",
+    "customerNumber": "1299453",
+    "contractNumber": "861418"
+  },
+  "lineItems": [
+    {
+      "description": "Waoo Fiber 1000",
+      "period": "01.09.2025 - 30.09.2025",
+      "amount": 389.00
+    },
+    {
+      "description": "Waoo Fiber 1000",
+      "period": "01.10.2025 - 31.10.2025",
+      "amount": 389.00
+    },
+    {
+      "description": "Waoo Fiber 1000",
+      "period": "01.11.2025 - 30.11.2025",
+      "amount": 389.00
+    },
+    {
+      "description": "Abonnement Fast IP adresse",
+      "period": "01.09.2025 - 30.09.2025",
+      "amount": 39.00
+    },
+    {
+      "description": "Abonnement Fast IP adresse",
+      "period": "01.10.2025 - 31.10.2025",
+      "amount": 39.00
+    },
+    {
+      "description": "Abonnement Fast IP adresse",
+      "period": "01.11.2025 - 30.11.2025",
+      "amount": 39.00
+    }
+  ],
+  "suggestedAccount": {
+    "number": 7300,
+    "name": "Internet og webhotel",
+    "vatCode": "I25",
+    "region": "DK",
+    "confidence": 0.98,
+    "reasoning": "Fiber internet abonnement fra dansk udbyder med 25% moms"
+  },
+  "alternativeAccounts": []
+}
--- a/account-suggestions/test-invoices/fibia_receipt_72147924.pdf
+++ b/account-suggestions/test-invoices/fibia_receipt_72147924.pdf
--- a/account-suggestions/test-invoices/invoice
+++ b/account-suggestions/test-invoices/invoice
--- a/account-suggestions/test-invoices/invoice-F175490.pdf
+++ b/account-suggestions/test-invoices/invoice-F175490.pdf
--- a/account-suggestions/test-invoices/invoice-F191023.pdf
+++ b/account-suggestions/test-invoices/invoice-F191023.pdf
--- a/account-suggestions/test-invoices/parking_receipt_2582441883.pdf
+++ b/account-suggestions/test-invoices/parking_receipt_2582441883.pdf
--- a/account-suggestions/test-invoices/receipt.png
+++ b/account-suggestions/test-invoices/receipt.png
--- a/backend/BACKEND_REQUIREMENTS.md
+++ b/backend/BACKEND_REQUIREMENTS.md
@ -0,0 +1,355 @@
+# Backend Krav - Dansk Bogføringssystem
+
+## Overblik
+
+Dette dokument beskriver backend-kravene for det danske bogføringssystem. Frontend er implementeret i React/TypeScript med Ant Design og Zustand state management.
+
+---
+
+## 1. Regnskabsår (Fiscal Years)
+
+### 1.1 Data Model
+
+```typescript
+interface FiscalYear {
+  id: string;
+  companyId: string;
+  name: string;                    // "2025" eller "2024/2025"
+  startDate: string;               // ISO date "YYYY-MM-DD"
+  endDate: string;                 // ISO date "YYYY-MM-DD"
+  status: 'open' | 'closed' | 'locked';
+  openingBalancePosted: boolean;
+  closingDate?: string;            // Når året blev lukket
+  closedBy?: string;               // Bruger ID
+  createdAt: string;
+  updatedAt: string;
+}
+```
+
+### 1.2 API Endpoints
+
+| Endpoint | Metode | Beskrivelse |
+|----------|--------|-------------|
+| `/api/fiscal-years` | GET | Hent alle regnskabsår for aktiv virksomhed |
+| `/api/fiscal-years` | POST | Opret nyt regnskabsår |
+| `/api/fiscal-years/:id` | GET | Hent specifikt regnskabsår |
+| `/api/fiscal-years/:id` | PATCH | Opdater regnskabsår |
+| `/api/fiscal-years/:id/close` | POST | Luk regnskabsår (årsafslutning) |
+| `/api/fiscal-years/:id/reopen` | POST | Genåbn lukket regnskabsår |
+| `/api/fiscal-years/:id/lock` | POST | Lås regnskabsår permanent |
+
+### 1.3 Forretningsregler
+
+#### Oprettelse
+- Nye regnskabsår må IKKE overlappe med eksisterende
+- Regnskabsår der "rører" ved grænsen (slutter/starter samme dag) er TILLADT
+- Understøt både kalenderår (jan-dec) og skæve regnskabsår (fx jul-jun)
+- Valider at regnskabsåret er mellem 300-400 dage (advarsel, ikke fejl)
+
+#### Årsafslutning (Year-End Closing)
+**KRITISK**: Ved årsafslutning skal backend:
+
+1. **Validere** at regnskabsåret kan lukkes:
+   - Ikke allerede låst
+   - Advar om åbne perioder (men tillad lukning)
+   - Advar om ikke-afstemte transaktioner
+
+2. **Oprette lukkeposter** (closing entries):
+   - Luk alle indtægtskonti til resultatoverførselskonto
+   - Luk alle udgiftskonti til resultatoverførselskonto
+   - Gem som normale transaktioner med særlig markering (`isClosingEntry: true`)
+
+3. **Generere åbningsbalancer** til næste regnskabsår:
+   - Beregn slutsaldo for alle balancekonti (aktiver, passiver, egenkapital)
+   - Nulstil resultatkonti (indtægter, udgifter, vareforbrug, personale, finansielle)
+   - Opret åbningsposteringer i næste regnskabsår (`isOpeningBalance: true`)
+
+4. **Opdatere status**:
+   - Sæt regnskabsår status til 'closed' eller 'locked'
+   - Luk alle tilhørende regnskabsperioder
+   - Sæt `openingBalancePosted: true` på næste regnskabsår
+
+#### Genåbning
+- Kun 'closed' regnskabsår kan genåbnes (ikke 'locked')
+- Ved genåbning skal åbningsbalancer i næste år opdateres automatisk
+- Log hvem der genåbnede og hvornår
+
+### 1.4 Dynamiske Åbningsbalancer
+
+Frontend forventer at åbningsbalancer opdateres automatisk når der bogføres i et tidligere år (som Dinero):
+
+```
+Scenarie:
+1. Regnskabsår 2024 er lukket, 2025 er åbent
+2. Revisor finder fejl og bogfører korrektion i 2024
+3. Backend skal automatisk genberegne åbningsbalancen for 2025
+```
+
+---
+
+## 2. Regnskabsperioder (Accounting Periods)
+
+### 2.1 Data Model
+
+```typescript
+interface AccountingPeriod {
+  id: string;
+  fiscalYearId: string;
+  companyId: string;
+  name: string;                    // "Januar 2025", "Q1 2025"
+  periodNumber: number;            // 1-12 for månedlig
+  startDate: string;
+  endDate: string;
+  status: 'future' | 'open' | 'closed' | 'locked';
+  closedAt?: string;
+  closedBy?: string;
+  reopenedAt?: string;
+  reopenedBy?: string;
+  lockedAt?: string;
+  lockedBy?: string;
+  createdAt: string;
+  updatedAt: string;
+}
+
+type PeriodFrequency = 'monthly' | 'quarterly' | 'half-yearly' | 'yearly';
+```
+
+### 2.2 API Endpoints
+
+| Endpoint | Metode | Beskrivelse |
+|----------|--------|-------------|
+| `/api/periods` | GET | Hent perioder (filtrer på fiscalYearId) |
+| `/api/periods/:id` | PATCH | Opdater periode |
+| `/api/periods/:id/close` | POST | Luk periode |
+| `/api/periods/:id/reopen` | POST | Genåbn periode |
+| `/api/periods/:id/lock` | POST | Lås periode |
+
+### 2.3 Forretningsregler
+
+- Perioder genereres automatisk ved oprettelse af regnskabsår
+- Understøt: månedlig (12), kvartalsvis (4), halvårlig (2), årlig (1)
+- Perioder kan kun lukkes i rækkefølge (periode 2 kræver periode 1 lukket)
+- Låste perioder kan IKKE genåbnes
+
+---
+
+## 3. Transaktioner med Periode-Reference
+
+### 3.1 Udvidet Transaction Model
+
+```typescript
+interface Transaction {
+  id: string;
+  companyId: string;
+  fiscalYearId: string;           // PÅKRÆVET - hvilket regnskabsår
+  periodId: string;               // PÅKRÆVET - hvilken periode
+  journalEntryNumber: number;
+  date: string;
+  description: string;
+  reference?: string;
+  lines: TransactionLine[];
+  isVoided: boolean;
+  isReconciled: boolean;
+  isClosingEntry?: boolean;       // NY - markering for lukkepost
+  isOpeningBalance?: boolean;     // NY - markering for åbningsbalance
+  createdAt: string;
+  updatedAt: string;
+  createdBy: string;
+}
+```
+
+### 3.2 Forretningsregler for Bogføring
+
+```typescript
+interface PostingValidation {
+  allowed: boolean;
+  reason?: string;
+  reasonDanish?: string;
+}
+```
+
+Backend skal validere ved bogføring:
+1. Find periode for transaktionsdato
+2. Tjek om perioden er åben
+3. Tjek om regnskabsåret er åbent
+4. Returner fejl med dansk besked hvis ikke tilladt
+
+---
+
+## 4. Kontoplan (Chart of Accounts)
+
+### 4.1 Data Model
+
+```typescript
+interface Account {
+  id: string;
+  companyId: string;
+  accountNumber: string;          // "1000", "3900"
+  name: string;
+  type: AccountType;
+  parentId?: string;
+  description?: string;
+  vatCodeId?: string;
+  isActive: boolean;
+  isSystemAccount: boolean;       // Kan ikke slettes
+  balance: number;                // Beregnet felt
+  createdAt: string;
+  updatedAt: string;
+}
+
+type AccountType =
+  | 'asset'        // Aktiver
+  | 'liability'    // Passiver
+  | 'equity'       // Egenkapital
+  | 'revenue'      // Indtægter
+  | 'cogs'         // Vareforbrug
+  | 'expense'      // Udgifter
+  | 'personnel'    // Personaleomkostninger
+  | 'financial'    // Finansielle poster
+  | 'extraordinary'; // Ekstraordinære poster
+```
+
+### 4.2 Systemkonti
+
+Følgende konti skal oprettes automatisk:
+- `3900` - Overført resultat (equity) - bruges til årsafslutning
+- Åbningsbalancekonto for primo-posteringer
+
+---
+
+## 5. Moms (VAT)
+
+### 5.1 Data Model
+
+```typescript
+interface VATCode {
+  id: string;
+  companyId: string;
+  code: string;                   // "S25", "K25"
+  name: string;
+  rate: number;                   // 0.25 for 25%
+  type: 'sales' | 'purchase' | 'eu_sales' | 'eu_purchase' | 'reverse_charge';
+  accountId: string;              // Momskonto
+  isActive: boolean;
+}
+
+interface VATPeriod {
+  id: string;
+  companyId: string;
+  startDate: string;
+  endDate: string;
+  dueDate: string;                // Indberetningsfrist
+  status: 'open' | 'submitted' | 'paid';
+  salesVAT: number;               // Beregnet
+  purchaseVAT: number;            // Beregnet
+  netVAT: number;                 // salesVAT - purchaseVAT
+  submittedAt?: string;
+  paidAt?: string;
+}
+```
+
+### 5.2 SKAT Integration (Fremtidig)
+
+- Momsindberetning til SKAT via NemVirksomhed API
+- Kvartalsvise eller månedlige momsperioder baseret på virksomhedsstørrelse
+
+---
+
+## 6. Virksomhed (Company)
+
+### 6.1 Data Model
+
+```typescript
+interface Company {
+  id: string;
+  name: string;
+  cvr: string;                    // Dansk CVR-nummer
+  address?: string;
+  postalCode?: string;
+  city?: string;
+  country: string;                // Default "DK"
+  fiscalYearStart: number;        // Måned 1-12 (default 1 for januar)
+  currency: string;               // Default "DKK"
+  vatRegistered: boolean;
+  vatPeriodFrequency: 'monthly' | 'quarterly' | 'half-yearly';
+  createdAt: string;
+  updatedAt: string;
+}
+```
+
+---
+
+## 7. Fejlhåndtering
+
+Alle fejlbeskeder skal være tilgængelige på både engelsk og dansk:
+
+```typescript
+interface APIError {
+  code: string;
+  message: string;
+  messageDanish: string;
+  details?: Record<string, unknown>;
+}
+```
+
+### Eksempler på fejlkoder
+
+| Kode | Engelsk | Dansk |
+|------|---------|-------|
+| `PERIOD_LOCKED` | Period is locked | Perioden er låst |
+| `FISCAL_YEAR_LOCKED` | Fiscal year is locked | Regnskabsåret er låst |
+| `OVERLAP_EXISTS` | Overlaps with existing fiscal year | Overlapper med eksisterende regnskabsår |
+| `UNBALANCED_ENTRY` | Debit and credit must be equal | Debet og kredit skal være ens |
+| `NO_COMPANY_SELECTED` | No company selected | Ingen virksomhed valgt |
+
+---
+
+## 8. Prioriteret Implementeringsrækkefølge
+
+### Fase 1: Grundlæggende (MVP)
+1. Company CRUD
+2. Account CRUD med standard kontoplan
+3. Transaction CRUD med validering
+4. FiscalYear CRUD
+
+### Fase 2: Periode-management
+1. AccountingPeriod generering og CRUD
+2. Periode-validering ved bogføring
+3. Periode lukning/åbning
+
+### Fase 3: Årsafslutning
+1. Closing entries generering og bogføring
+2. Opening balance beregning og bogføring
+3. Dynamisk åbningsbalance-opdatering
+
+### Fase 4: Moms
+1. VATCode CRUD
+2. VATPeriod generering
+3. Momsberegning og -rapportering
+
+---
+
+## 9. Tekniske Krav
+
+### Authentication
+- JWT-baseret authentication
+- Multi-tenant support (bruger kan have adgang til flere virksomheder)
+
+### Database
+- PostgreSQL anbefales
+- Soft delete på alle entiteter
+- Audit trail (createdBy, updatedBy, deletedBy)
+
+### API Format
+- RESTful JSON API
+- GraphQL som alternativ (valgfrit)
+- Pagination på liste-endpoints
+- Filtering og sorting support
+
+### Valuta
+- Alle beløb i øre/cents (integers) for at undgå floating point problemer
+- Frontend konverterer til/fra DKK ved visning
+
+---
+
+*Sidst opdateret: 17. januar 2026*
--- a/backend/Books.Api.Tests/Books.Api.Tests.csproj
+++ b/backend/Books.Api.Tests/Books.Api.Tests.csproj
@ -0,0 +1,37 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>14</LangVersion>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <!-- Test Framework -->
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+
+    <!-- Integration Testing -->
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.0" />
+    <PackageReference Include="FluentAssertions" Version="8.3.0" />
+
+    <!-- Hangfire In-Memory for Tests -->
+    <PackageReference Include="Hangfire.InMemory" Version="1.0.0" />
+
+    <!-- PostgreSQL for Test Database -->
+    <PackageReference Include="Npgsql" Version="10.0.1" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Books.Api\Books.Api.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+</Project>
--- a/backend/Books.Api.Tests/GraphQL/CompanyGraphQLTests.cs
+++ b/backend/Books.Api.Tests/GraphQL/CompanyGraphQLTests.cs
@ -0,0 +1,260 @@
+using Books.Api.Domain.Companies;
+using Books.Api.EventFlow.Repositories;
+using Books.Api.Tests.Helpers;
+using Books.Api.Tests.Infrastructure;
+using FluentAssertions;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Books.Api.Tests.GraphQL;
+
+/// <summary>
+/// Integration tests for Company GraphQL operations.
+/// Each test class runs with its own isolated database.
+/// </summary>
+[Trait("Category", "Integration")]
+public class CompanyGraphQLTests(TestWebApplicationFactory factory)
+    : IntegrationTestBase(factory)
+{
+    [Fact]
+    public async Task Query_Companies_ReturnsEmptyList_WhenNoCompaniesExist()
+    {
+        // Arrange
+        var graphqlClient = new GraphQLTestClient(Client);
+
+        // Act
+        var response = await graphqlClient.QueryAsync<CompaniesResponse>("""
+            query {
+                companies {
+                    id
+                    name
+                }
+            }
+            """);
+
+        // Assert
+        response.EnsureNoErrors();
+        response.Data.Should().NotBeNull();
+        response.Data!.Companies.Should().BeEmpty();
+    }
+
+    [Fact]
+    public async Task Mutation_CreateCompany_CreatesCompanySuccessfully()
+    {
+        // Arrange
+        var graphqlClient = new GraphQLTestClient(Client);
+
+        // Act
+        var response = await graphqlClient.MutateAsync<CreateCompanyResponse>("""
+            mutation CreateCompany($input: CreateCompanyInput!) {
+                createCompany(input: $input) {
+                    id
+                    name
+                    cvr
+                    country
+                    currency
+                    fiscalYearStartMonth
+                }
+            }
+            """,
+            new
+            {
+                input = new
+                {
+                    name = "Test Virksomhed A/S",
+                    cvr = "12345678",
+                    country = "DK",
+                    currency = "DKK",
+                    fiscalYearStartMonth = 1
+                }
+            });
+
+        // Assert
+        response.EnsureNoErrors();
+        response.Data.Should().NotBeNull();
+        response.Data!.CreateCompany.Should().NotBeNull();
+        response.Data.CreateCompany!.Name.Should().Be("Test Virksomhed A/S");
+        response.Data.CreateCompany.Cvr.Should().Be("12345678");
+        response.Data.CreateCompany.Country.Should().Be("DK");
+        response.Data.CreateCompany.Currency.Should().Be("DKK");
+        response.Data.CreateCompany.FiscalYearStartMonth.Should().Be(1);
+    }
+
+    [Fact]
+    public async Task Query_Company_ReturnsCompany_AfterCreation()
+    {
+        // Arrange
+        var graphqlClient = new GraphQLTestClient(Client);
+
+        // Create a company first
+        var createResponse = await graphqlClient.MutateAsync<CreateCompanyResponse>("""
+            mutation CreateCompany($input: CreateCompanyInput!) {
+                createCompany(input: $input) {
+                    id
+                    name
+                }
+            }
+            """,
+            new
+            {
+                input = new
+                {
+                    name = "Query Test Virksomhed",
+                    cvr = "87654321"
+                }
+            });
+
+        createResponse.EnsureNoErrors();
+        var companyId = createResponse.Data!.CreateCompany!.Id;
+
+        // Act - Query the company by ID (with eventual consistency)
+        var company = await Eventually.GetAsync(async () =>
+        {
+            var response = await graphqlClient.QueryAsync<CompanyResponse>("""
+                query GetCompany($id: ID!) {
+                    company(id: $id) {
+                        id
+                        name
+                        cvr
+                    }
+                }
+                """,
+                new { id = companyId });
+
+            return response.Data?.Company;
+        });
+
+        // Assert
+        company.Should().NotBeNull();
+        company!.Name.Should().Be("Query Test Virksomhed");
+        company.Cvr.Should().Be("87654321");
+    }
+
+    [Fact]
+    public async Task Query_Companies_ReturnsAllCompanies_AfterMultipleCreations()
+    {
+        // Arrange
+        var graphqlClient = new GraphQLTestClient(Client);
+
+        // Create multiple companies with unique names for this test
+        var company1Name = $"ListTest-Virksomhed-{Guid.NewGuid():N}";
+        var company2Name = $"ListTest-Virksomhed-{Guid.NewGuid():N}";
+
+        await graphqlClient.MutateAsync<CreateCompanyResponse>($$"""
+            mutation { createCompany(input: { name: "{{company1Name}}" }) { id } }
+            """);
+
+        await graphqlClient.MutateAsync<CreateCompanyResponse>($$"""
+            mutation { createCompany(input: { name: "{{company2Name}}" }) { id } }
+            """);
+
+        // Act - Wait for eventual consistency (at least 2 companies with our prefix)
+        var companies = await Eventually.GetAsync(
+            async () =>
+            {
+                var response = await graphqlClient.QueryAsync<CompaniesResponse>("""
+                    query {
+                        companies {
+                            id
+                            name
+                        }
+                    }
+                    """);
+                var allCompanies = response.Data?.Companies ?? [];
+                var ourCompanies = allCompanies.Where(c => c.Name.StartsWith("ListTest-")).ToList();
+                return ourCompanies.Count >= 2 ? ourCompanies : null;
+            });
+
+        // Assert - verify our specific companies are present
+        companies.Should().Contain(c => c.Name == company1Name);
+        companies.Should().Contain(c => c.Name == company2Name);
+    }
+
+    [Fact]
+    public async Task Mutation_UpdateCompany_UpdatesCompanySuccessfully()
+    {
+        // Arrange
+        var graphqlClient = new GraphQLTestClient(Client);
+
+        // Create a company first
+        var createResponse = await graphqlClient.MutateAsync<CreateCompanyResponse>("""
+            mutation { createCompany(input: { name: "Original Name" }) { id } }
+            """);
+
+        createResponse.EnsureNoErrors();
+        var companyId = createResponse.Data!.CreateCompany!.Id;
+
+        // Wait for the company to be created
+        await Eventually.GetAsync(async () =>
+        {
+            var repository = GetService<ICompanyRepository>();
+            return await repository.GetByIds([CompanyId.With(companyId)]);
+        });
+
+        // Act - Update the company
+        var updateResponse = await graphqlClient.MutateAsync<UpdateCompanyResponse>("""
+            mutation UpdateCompany($id: ID!, $input: UpdateCompanyInput!) {
+                updateCompany(id: $id, input: $input) {
+                    id
+                    name
+                    cvr
+                }
+            }
+            """,
+            new
+            {
+                id = companyId,
+                input = new
+                {
+                    name = "Updated Name",
+                    cvr = "11111111"
+                }
+            });
+
+        // Assert
+        updateResponse.EnsureNoErrors();
+
+        // Wait for eventual consistency and verify update
+        var updatedCompany = await Eventually.GetAsync(async () =>
+        {
+            var repository = GetService<ICompanyRepository>();
+            var companies = await repository.GetByIds([CompanyId.With(companyId)]);
+            var company = companies.First();
+            return company?.Name == "Updated Name" ? company : null;
+        });
+
+        updatedCompany.Should().NotBeNull();
+        updatedCompany!.Name.Should().Be("Updated Name");
+        updatedCompany.Cvr.Should().Be("11111111");
+    }
+
+    // Response DTOs for deserialization
+    private class CompaniesResponse
+    {
+        public List<CompanyDto> Companies { get; set; } = [];
+    }
+
+    private class CompanyResponse
+    {
+        public CompanyDto? Company { get; set; }
+    }
+
+    private class CreateCompanyResponse
+    {
+        public CompanyDto? CreateCompany { get; set; }
+    }
+
+    private class UpdateCompanyResponse
+    {
+        public CompanyDto? UpdateCompany { get; set; }
+    }
+
+    private class CompanyDto
+    {
+        public string Id { get; set; } = string.Empty;
+        public string Name { get; set; } = string.Empty;
+        public string? Cvr { get; set; }
+        public string? Country { get; set; }
+        public string? Currency { get; set; }
+        public int? FiscalYearStartMonth { get; set; }
+    }
+}
--- a/backend/Books.Api.Tests/Helpers/Eventually.cs
+++ b/backend/Books.Api.Tests/Helpers/Eventually.cs
@ -0,0 +1,89 @@
+namespace Books.Api.Tests.Helpers;
+
+/// <summary>
+/// Helper for testing eventually consistent systems.
+/// Polls a condition until it's met or times out.
+/// </summary>
+public static class Eventually
+{
+    /// <summary>
+    /// Polls a condition until it returns true or times out.
+    /// </summary>
+    public static async Task AssertAsync(
+        Func<Task<bool>> condition,
+        TimeSpan? timeout = null,
+        TimeSpan? pollInterval = null,
+        string? failMessage = null)
+    {
+        timeout ??= TimeSpan.FromSeconds(5);
+        pollInterval ??= TimeSpan.FromMilliseconds(50);
+
+        var start = DateTime.UtcNow;
+
+        while (DateTime.UtcNow - start < timeout)
+        {
+            if (await condition())
+                return;
+
+            await Task.Delay(pollInterval.Value);
+        }
+
+        throw new TimeoutException(
+            failMessage ?? $"Condition was not met within {timeout}.");
+    }
+
+    /// <summary>
+    /// Polls until a non-null result is returned or times out.
+    /// </summary>
+    public static async Task<T> GetAsync<T>(
+        Func<Task<T?>> getter,
+        TimeSpan? timeout = null,
+        TimeSpan? pollInterval = null,
+        string? failMessage = null) where T : class
+    {
+        timeout ??= TimeSpan.FromSeconds(10);
+        pollInterval ??= TimeSpan.FromMilliseconds(100);
+
+        var start = DateTime.UtcNow;
+
+        while (DateTime.UtcNow - start < timeout)
+        {
+            var result = await getter();
+            if (result != null)
+                return result;
+
+            await Task.Delay(pollInterval.Value);
+        }
+
+        throw new TimeoutException(
+            failMessage ?? $"Expected non-null result was not returned within {timeout}.");
+    }
+
+    /// <summary>
+    /// Polls until a collection has the expected count or times out.
+    /// </summary>
+    public static async Task<IReadOnlyList<T>> GetListAsync<T>(
+        Func<Task<IReadOnlyList<T>>> getter,
+        int expectedCount,
+        TimeSpan? timeout = null,
+        TimeSpan? pollInterval = null,
+        string? failMessage = null)
+    {
+        timeout ??= TimeSpan.FromSeconds(10);
+        pollInterval ??= TimeSpan.FromMilliseconds(100);
+
+        var start = DateTime.UtcNow;
+
+        while (DateTime.UtcNow - start < timeout)
+        {
+            var result = await getter();
+            if (result.Count >= expectedCount)
+                return result;
+
+            await Task.Delay(pollInterval.Value);
+        }
+
+        throw new TimeoutException(
+            failMessage ?? $"Expected {expectedCount} items but condition was not met within {timeout}.");
+    }
+}
--- a/backend/Books.Api.Tests/Helpers/GraphQLTestClient.cs
+++ b/backend/Books.Api.Tests/Helpers/GraphQLTestClient.cs
@ -0,0 +1,125 @@
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Books.Api.Tests.Helpers;
+
+/// <summary>
+/// Helper client for executing GraphQL queries and mutations in tests.
+/// </summary>
+public class GraphQLTestClient(HttpClient httpClient)
+{
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        PropertyNameCaseInsensitive = true,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
+    };
+
+    /// <summary>
+    /// Executes a GraphQL query and returns the typed result.
+    /// </summary>
+    public async Task<GraphQLResponse<T>> QueryAsync<T>(
+        string query,
+        object? variables = null,
+        CancellationToken cancellationToken = default)
+    {
+        return await ExecuteAsync<T>(query, variables, cancellationToken);
+    }
+
+    /// <summary>
+    /// Executes a GraphQL mutation and returns the typed result.
+    /// </summary>
+    public async Task<GraphQLResponse<T>> MutateAsync<T>(
+        string mutation,
+        object? variables = null,
+        CancellationToken cancellationToken = default)
+    {
+        return await ExecuteAsync<T>(mutation, variables, cancellationToken);
+    }
+
+    /// <summary>
+    /// Executes a GraphQL operation and returns the raw JSON response.
+    /// </summary>
+    public async Task<string> ExecuteRawAsync(
+        string query,
+        object? variables = null,
+        CancellationToken cancellationToken = default)
+    {
+        var request = new GraphQLRequest(query, variables);
+        var json = JsonSerializer.Serialize(request, JsonOptions);
+        var content = new StringContent(json, Encoding.UTF8, "application/json");
+
+        var response = await httpClient.PostAsync("/graphql", content, cancellationToken);
+        return await response.Content.ReadAsStringAsync(cancellationToken);
+    }
+
+    private async Task<GraphQLResponse<T>> ExecuteAsync<T>(
+        string query,
+        object? variables,
+        CancellationToken cancellationToken)
+    {
+        var request = new GraphQLRequest(query, variables);
+        var json = JsonSerializer.Serialize(request, JsonOptions);
+        var content = new StringContent(json, Encoding.UTF8, "application/json");
+
+        var response = await httpClient.PostAsync("/graphql", content, cancellationToken);
+        var responseJson = await response.Content.ReadAsStringAsync(cancellationToken);
+
+        var result = JsonSerializer.Deserialize<GraphQLResponse<T>>(responseJson, JsonOptions);
+        return result ?? throw new InvalidOperationException("Failed to deserialize GraphQL response");
+    }
+}
+
+public record GraphQLRequest(
+    [property: JsonPropertyName("query")] string Query,
+    [property: JsonPropertyName("variables")] object? Variables = null);
+
+public class GraphQLResponse<T>
+{
+    [JsonPropertyName("data")]
+    public T? Data { get; set; }
+
+    [JsonPropertyName("errors")]
+    public List<GraphQLError>? Errors { get; set; }
+
+    public bool HasErrors => Errors?.Count > 0;
+
+    public void EnsureNoErrors()
+    {
+        if (HasErrors)
+        {
+            var errorDetails = Errors!.Select(e =>
+            {
+                var msg = e.Message;
+                if (e.Extensions != null)
+                {
+                    foreach (var ext in e.Extensions)
+                    {
+                        msg += $"{Environment.NewLine}  [{ext.Key}]: {ext.Value}";
+                    }
+                }
+                return msg;
+            });
+            var messages = string.Join(Environment.NewLine, errorDetails);
+            throw new GraphQLException($"GraphQL errors occurred:{Environment.NewLine}{messages}", Errors!);
+        }
+    }
+}
+
+public class GraphQLError
+{
+    [JsonPropertyName("message")]
+    public string Message { get; set; } = string.Empty;
+
+    [JsonPropertyName("path")]
+    public List<object>? Path { get; set; }
+
+    [JsonPropertyName("extensions")]
+    public Dictionary<string, object>? Extensions { get; set; }
+}
+
+public class GraphQLException(string message, List<GraphQLError> errors) : Exception(message)
+{
+    public List<GraphQLError> Errors { get; } = errors;
+}
--- a/backend/Books.Api.Tests/Infrastructure/IntegrationTestBase.cs
+++ b/backend/Books.Api.Tests/Infrastructure/IntegrationTestBase.cs
@ -0,0 +1,38 @@
+using EventFlow;
+using EventFlow.Aggregates;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Books.Api.Tests.Infrastructure;
+
+/// <summary>
+/// Base class for integration tests with isolated database per test class.
+/// Uses IClassFixture pattern - each test class gets its own database.
+/// </summary>
+public abstract class IntegrationTestBase
+    : IClassFixture<TestWebApplicationFactory>, IDisposable
+{
+    protected readonly TestWebApplicationFactory Factory;
+    protected readonly HttpClient Client;
+    private readonly IServiceScope _scope;
+
+    protected IntegrationTestBase(TestWebApplicationFactory factory)
+    {
+        Factory = factory;
+        Client = factory.CreateGraphQLClient();
+        _scope = factory.Services.CreateScope();
+    }
+
+    protected IServiceProvider Services => _scope.ServiceProvider;
+
+    protected ICommandBus CommandBus => Services.GetRequiredService<ICommandBus>();
+    protected IAggregateStore AggregateStore => Services.GetRequiredService<IAggregateStore>();
+
+    protected T GetService<T>() where T : notnull
+        => Services.GetRequiredService<T>();
+
+    public void Dispose()
+    {
+        _scope?.Dispose();
+        GC.SuppressFinalize(this);
+    }
+}
--- a/backend/Books.Api.Tests/Infrastructure/TestDatabase.cs
+++ b/backend/Books.Api.Tests/Infrastructure/TestDatabase.cs
@ -0,0 +1,121 @@
+using Npgsql;
+
+namespace Books.Api.Tests.Infrastructure;
+
+/// <summary>
+/// Creates an isolated PostgreSQL database for each test run.
+/// The database is automatically dropped when disposed.
+/// </summary>
+public class TestDatabase : IDisposable
+{
+    public const string DefaultConnectionString =
+        "Host=localhost;Port=5432;Username=postgres;Password=postgres;Database=postgres;Include Error Detail=true";
+
+    private readonly string _masterConnectionString;
+    private bool _disposed;
+
+    public string DatabaseName { get; }
+
+    public string ConnectionString
+    {
+        get
+        {
+            var builder = new NpgsqlConnectionStringBuilder(_masterConnectionString)
+            {
+                Database = DatabaseName
+            };
+            return builder.ToString();
+        }
+    }
+
+    public TestDatabase(string? masterConnectionString = null)
+    {
+        _masterConnectionString = masterConnectionString ?? GetConnectionStringFromEnvironment();
+        DatabaseName = $"books_test_{Guid.NewGuid():N}";
+
+        CreateDatabase();
+    }
+
+    private static string GetConnectionStringFromEnvironment()
+    {
+        return Environment.GetEnvironmentVariable("TEST_CONNECTION_STRING")
+            ?? DefaultConnectionString;
+    }
+
+    private void CreateDatabase()
+    {
+        try
+        {
+            using var connection = new NpgsqlConnection(_masterConnectionString);
+            connection.Open();
+
+            using var command = connection.CreateCommand();
+            command.CommandText = $"CREATE DATABASE \"{DatabaseName}\"";
+            command.ExecuteNonQuery();
+
+            Console.WriteLine($"[TestDatabase] Created test database: {DatabaseName}");
+        }
+        catch (Exception e)
+        {
+            throw new InvalidOperationException(
+                $"Failed to create test database '{DatabaseName}' using connection string: {_masterConnectionString}",
+                e);
+        }
+    }
+
+    public void Dispose()
+    {
+        Dispose(true);
+        GC.SuppressFinalize(this);
+    }
+
+    protected virtual void Dispose(bool disposing)
+    {
+        if (_disposed) return;
+
+        if (disposing)
+        {
+            try
+            {
+                // Step 1: Terminate all active connections to prevent "database in use" errors
+                using (var connection = new NpgsqlConnection(_masterConnectionString))
+                {
+                    connection.Open();
+
+                    using var terminateCommand = connection.CreateCommand();
+                    terminateCommand.CommandText = $@"
+                        SELECT pg_terminate_backend(pg_stat_activity.pid)
+                        FROM pg_stat_activity
+                        WHERE pg_stat_activity.datname = '{DatabaseName}'
+                          AND pid <> pg_backend_pid();
+                    ";
+                    terminateCommand.ExecuteNonQuery();
+                }
+
+                // Step 2: Drop the database
+                using (var connection = new NpgsqlConnection(_masterConnectionString))
+                {
+                    connection.Open();
+
+                    using var dropCommand = connection.CreateCommand();
+                    dropCommand.CommandText = $"DROP DATABASE IF EXISTS \"{DatabaseName}\"";
+                    dropCommand.ExecuteNonQuery();
+                }
+
+                Console.WriteLine($"[TestDatabase] Dropped test database: {DatabaseName}");
+            }
+            catch (Exception e)
+            {
+                // Log but don't throw - we don't want cleanup failures to mask test failures
+                Console.WriteLine($"[TestDatabase] Warning: Failed to drop test database {DatabaseName}: {e.Message}");
+            }
+        }
+
+        _disposed = true;
+    }
+
+    ~TestDatabase()
+    {
+        Dispose(false);
+    }
+}
--- a/backend/Books.Api.Tests/Infrastructure/TestWebApplicationFactory.cs
+++ b/backend/Books.Api.Tests/Infrastructure/TestWebApplicationFactory.cs
@ -0,0 +1,146 @@
+using Books.Api.EventFlow.Infrastructure;
+using EventFlow.PostgreSql;
+using EventFlow.PostgreSql.Connections;
+using EventFlow.PostgreSql.EventStores;
+using Hangfire;
+using Hangfire.InMemory;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc.Testing;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Hosting;
+using Npgsql;
+
+namespace Books.Api.Tests.Infrastructure;
+
+/// <summary>
+/// WebApplicationFactory configured for integration testing with:
+/// - Isolated database per test class
+/// - In-memory Hangfire (no real job processing)
+/// - Full GraphQL endpoint support
+/// </summary>
+public class TestWebApplicationFactory : WebApplicationFactory<Program>
+{
+    static TestWebApplicationFactory()
+    {
+        // Enable legacy timestamp behavior for Npgsql 6.0+
+        // Must be set before any Npgsql connections are created
+        AppContext.SetSwitch("Npgsql.EnableLegacyTimestampBehavior", true);
+    }
+
+    private readonly TestDatabase _testDatabase = new();
+
+    public string TestDatabaseName => _testDatabase.DatabaseName;
+    public string ConnectionString => _testDatabase.ConnectionString;
+
+    protected override void ConfigureWebHost(IWebHostBuilder builder)
+    {
+        // Set environment to Test to avoid Development-only middleware (like Altair)
+        builder.UseEnvironment("Test");
+
+        builder.ConfigureAppConfiguration((context, config) =>
+        {
+            // Clear all default configuration sources
+            config.Sources.Clear();
+
+            // Add test configuration with isolated database
+            var testConfig = new Dictionary<string, string?>
+            {
+                ["ConnectionStrings:Default"] = _testDatabase.ConnectionString,
+                ["AllowedHosts"] = "*"
+            };
+
+            config.AddInMemoryCollection(testConfig);
+
+            Console.WriteLine($"[TestWebApplicationFactory] Using test database: {_testDatabase.DatabaseName}");
+            Console.WriteLine($"[TestWebApplicationFactory] Connection string: {_testDatabase.ConnectionString}");
+        });
+
+        builder.ConfigureServices(services =>
+        {
+            // Run database migrations on the test database BEFORE services are built
+            // This ensures the test database has the correct schema
+            MigrateTestDatabase();
+
+            // Replace the NpgsqlDataSource to use the test database
+            services.RemoveAll<NpgsqlDataSource>();
+            services.AddSingleton(new NpgsqlDataSourceBuilder(_testDatabase.ConnectionString).Build());
+
+            // CRITICAL: Replace EventFlow's PostgreSQL configuration with test connection string
+            // This ensures the event store uses the test database, not the original
+            services.RemoveAll<IPostgreSqlConfiguration>();
+            services.AddSingleton<IPostgreSqlConfiguration>(
+                PostgreSqlConfiguration.New.SetConnectionString(_testDatabase.ConnectionString));
+
+            // Replace only Hangfire storage (not EventFlow.Hangfire integration services)
+            // We keep EventFlow.Hangfire services like IQueueNameProvider, HangfireJobScheduler
+            var hangfireStorageDescriptors = services
+                .Where(d =>
+                    d.ServiceType.FullName?.Contains("Hangfire.JobStorage") == true ||
+                    d.ServiceType.FullName?.Contains("Hangfire.Server.BackgroundJobServer") == true ||
+                    d.ImplementationType?.FullName?.Contains("Hangfire.PostgreSql") == true)
+                .ToList();
+
+            foreach (var descriptor in hangfireStorageDescriptors)
+            {
+                services.Remove(descriptor);
+            }
+
+            // Add Hangfire with in-memory storage
+            services.AddHangfire(config => config
+                .SetDataCompatibilityLevel(CompatibilityLevel.Version_180)
+                .UseSimpleAssemblyNameTypeSerializer()
+                .UseRecommendedSerializerSettings()
+                .UseInMemoryStorage());
+
+            services.AddHangfireServer(options =>
+            {
+                options.WorkerCount = 1;
+                options.Queues = ["default"];
+            });
+        });
+    }
+
+    protected override IHost CreateHost(IHostBuilder builder)
+    {
+        var host = base.CreateHost(builder);
+
+        // Run EventFlow PostgreSQL migrations after host is built
+        using var scope = host.Services.CreateScope();
+        var eventFlowMigrator = scope.ServiceProvider.GetRequiredService<IPostgreSqlDatabaseMigrator>();
+        EventFlowEventStoresPostgreSql.MigrateDatabaseAsync(eventFlowMigrator, CancellationToken.None).Wait();
+
+        return host;
+    }
+
+    private void MigrateTestDatabase()
+    {
+        // Run DbUp migrations for read model tables
+        DatabaseMigrator.Migrate(_testDatabase.ConnectionString);
+    }
+
+    protected override void Dispose(bool disposing)
+    {
+        if (disposing)
+        {
+            // Reset Hangfire's static LogProvider before disposing to prevent
+            // ObjectDisposedException when the next test class creates a new factory.
+            Hangfire.Logging.LogProvider.SetCurrentLogProvider(null);
+
+            _testDatabase.Dispose();
+        }
+
+        base.Dispose(disposing);
+    }
+
+    /// <summary>
+    /// Creates an HttpClient configured for GraphQL requests.
+    /// </summary>
+    public HttpClient CreateGraphQLClient()
+    {
+        var client = CreateClient();
+        client.BaseAddress = new Uri("http://localhost/graphql");
+        return client;
+    }
+}
--- a/backend/Books.Api.Tests/UnitTest1.cs
+++ b/backend/Books.Api.Tests/UnitTest1.cs
@ -0,0 +1,10 @@
+namespace Books.Api.Tests;
+
+public class UnitTest1
+{
+    [Fact]
+    public void Test1()
+    {
+
+    }
+}
--- a/backend/Books.Api.Tests/xunit.runner.json
+++ b/backend/Books.Api.Tests/xunit.runner.json
@ -0,0 +1,5 @@
+{
+  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+  "parallelizeAssembly": false,
+  "parallelizeTestCollections": false
+}
--- a/backend/Books.Api/Books.Api.csproj
+++ b/backend/Books.Api/Books.Api.csproj
@ -0,0 +1,46 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>14</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <AnalysisLevel>latest</AnalysisLevel>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <!-- EventFlow (local source) -->
+    <ProjectReference Include="../../../EventFlow/Source/EventFlow/EventFlow.csproj" />
+    <ProjectReference Include="../../../EventFlow/Source/EventFlow.PostgreSql/EventFlow.PostgreSql.csproj" />
+    <ProjectReference Include="../../../EventFlow/Source/EventFlow.Hangfire/EventFlow.Hangfire.csproj" />
+
+    <!-- Database -->
+    <PackageReference Include="Npgsql" Version="10.0.1" />
+    <PackageReference Include="Dapper" Version="2.1.66" />
+    <PackageReference Include="dbup-postgresql" Version="5.0.40" />
+
+    <!-- Hangfire -->
+    <PackageReference Include="Hangfire.Core" Version="1.8.22" />
+    <PackageReference Include="Hangfire.AspNetCore" Version="1.8.22" />
+    <PackageReference Include="Hangfire.PostgreSql" Version="1.20.13" />
+
+    <!-- Serialization -->
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
+
+    <!-- GraphQL -->
+    <PackageReference Include="GraphQL" Version="8.0.2" />
+    <PackageReference Include="GraphQL.Server.Transports.AspNetCore" Version="8.0.2" />
+    <PackageReference Include="GraphQL.Server.Ui.Altair" Version="8.0.2" />
+    <PackageReference Include="GraphQL.DataLoader" Version="8.0.2" />
+    <PackageReference Include="GraphQL.MicrosoftDI" Version="8.0.2" />
+
+    <!-- DI Decoration -->
+    <PackageReference Include="Scrutor" Version="5.0.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <EmbeddedResource Include="Database\Migrations\*.sql" />
+  </ItemGroup>
+
+</Project>
--- a/backend/Books.Api/Books.Api.http
+++ b/backend/Books.Api/Books.Api.http
@ -0,0 +1,6 @@
+@Books.Api_HostAddress = http://localhost:5142
+
+GET {{Books.Api_HostAddress}}/weatherforecast/
+Accept: application/json
+
+###
--- a/backend/Books.Api/Commands/Companies/CompanyCommandHandlers.cs
+++ b/backend/Books.Api/Commands/Companies/CompanyCommandHandlers.cs
@ -0,0 +1,50 @@
+using Books.Api.Domain.Companies;
+using EventFlow.Commands;
+
+namespace Books.Api.Commands.Companies;
+
+public class CreateCompanyCommandHandler : CommandHandler<CompanyAggregate, CompanyId, CreateCompanyCommand>
+{
+    public override Task ExecuteAsync(
+        CompanyAggregate aggregate,
+        CreateCompanyCommand command,
+        CancellationToken cancellationToken)
+    {
+        aggregate.Create(
+            command.Name,
+            command.Cvr,
+            command.Address,
+            command.PostalCode,
+            command.City,
+            command.Country,
+            command.FiscalYearStartMonth,
+            command.Currency,
+            command.VatRegistered,
+            command.VatPeriodFrequency);
+
+        return Task.CompletedTask;
+    }
+}
+
+public class UpdateCompanyCommandHandler : CommandHandler<CompanyAggregate, CompanyId, UpdateCompanyCommand>
+{
+    public override Task ExecuteAsync(
+        CompanyAggregate aggregate,
+        UpdateCompanyCommand command,
+        CancellationToken cancellationToken)
+    {
+        aggregate.Update(
+            command.Name,
+            command.Cvr,
+            command.Address,
+            command.PostalCode,
+            command.City,
+            command.Country,
+            command.FiscalYearStartMonth,
+            command.Currency,
+            command.VatRegistered,
+            command.VatPeriodFrequency);
+
+        return Task.CompletedTask;
+    }
+}
--- a/backend/Books.Api/Commands/Companies/CompanyCommands.cs
+++ b/backend/Books.Api/Commands/Companies/CompanyCommands.cs
@ -0,0 +1,56 @@
+using Books.Api.Domain.Companies;
+using EventFlow.Commands;
+
+namespace Books.Api.Commands.Companies;
+
+public class CreateCompanyCommand(
+    CompanyId aggregateId,
+    string name,
+    string? cvr,
+    string? address,
+    string? postalCode,
+    string? city,
+    string country,
+    int fiscalYearStartMonth,
+    string currency,
+    bool vatRegistered,
+    string? vatPeriodFrequency)
+    : Command<CompanyAggregate, CompanyId>(aggregateId)
+{
+    public string Name { get; } = name;
+    public string? Cvr { get; } = cvr;
+    public string? Address { get; } = address;
+    public string? PostalCode { get; } = postalCode;
+    public string? City { get; } = city;
+    public string Country { get; } = country;
+    public int FiscalYearStartMonth { get; } = fiscalYearStartMonth;
+    public string Currency { get; } = currency;
+    public bool VatRegistered { get; } = vatRegistered;
+    public string? VatPeriodFrequency { get; } = vatPeriodFrequency;
+}
+
+public class UpdateCompanyCommand(
+    CompanyId aggregateId,
+    string name,
+    string? cvr,
+    string? address,
+    string? postalCode,
+    string? city,
+    string country,
+    int fiscalYearStartMonth,
+    string currency,
+    bool vatRegistered,
+    string? vatPeriodFrequency)
+    : Command<CompanyAggregate, CompanyId>(aggregateId)
+{
+    public string Name { get; } = name;
+    public string? Cvr { get; } = cvr;
+    public string? Address { get; } = address;
+    public string? PostalCode { get; } = postalCode;
+    public string? City { get; } = city;
+    public string Country { get; } = country;
+    public int FiscalYearStartMonth { get; } = fiscalYearStartMonth;
+    public string Currency { get; } = currency;
+    public bool VatRegistered { get; } = vatRegistered;
+    public string? VatPeriodFrequency { get; } = vatPeriodFrequency;
+}
--- a/backend/Books.Api/Controllers/WeatherForecastController.cs
+++ b/backend/Books.Api/Controllers/WeatherForecastController.cs
@ -0,0 +1,25 @@
+using Microsoft.AspNetCore.Mvc;
+
+namespace Books.Api.Controllers;
+
+[ApiController]
+[Route("[controller]")]
+public class WeatherForecastController : ControllerBase
+{
+    private static readonly string[] Summaries =
+    [
+        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
+    ];
+
+    [HttpGet(Name = "GetWeatherForecast")]
+    public IEnumerable<WeatherForecast> Get()
+    {
+        return Enumerable.Range(1, 5).Select(index => new WeatherForecast
+        {
+            Date = DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
+            TemperatureC = Random.Shared.Next(-20, 55),
+            Summary = Summaries[Random.Shared.Next(Summaries.Length)]
+        })
+        .ToArray();
+    }
+}
--- a/backend/Books.Api/Database/Migrations/001_Initial.sql
+++ b/backend/Books.Api/Database/Migrations/001_Initial.sql
@ -0,0 +1,110 @@
+-- 001_Initial.sql
+-- Creates read model tables for Books API
+-- NOTE: Column names use snake_case (PostgreSQL convention)
+-- Custom ReadModelSqlGenerator converts C# PascalCase to snake_case
+
+-- Company read models
+CREATE TABLE IF NOT EXISTS company_read_models (
+    -- EventFlow standard columns
+    aggregate_id VARCHAR(255) PRIMARY KEY,
+    create_time TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_time TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    last_aggregate_sequence_number INT NOT NULL DEFAULT 1,
+
+    -- Business columns (snake_case)
+    name VARCHAR(255) NOT NULL,
+    cvr VARCHAR(8),
+    address VARCHAR(500),
+    postal_code VARCHAR(10),
+    city VARCHAR(100),
+    country VARCHAR(2) NOT NULL DEFAULT 'DK',
+    fiscal_year_start_month SMALLINT NOT NULL DEFAULT 1,
+    currency VARCHAR(3) NOT NULL DEFAULT 'DKK',
+    vat_registered BOOLEAN NOT NULL DEFAULT FALSE,
+    vat_period_frequency VARCHAR(20)
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_company_cvr ON company_read_models(cvr) WHERE cvr IS NOT NULL;
+
+-- Fiscal year read models
+CREATE TABLE IF NOT EXISTS fiscal_year_read_models (
+    aggregate_id VARCHAR(255) PRIMARY KEY,
+    create_time TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_time TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    last_aggregate_sequence_number INT NOT NULL DEFAULT 1,
+
+    company_id VARCHAR(255) NOT NULL,
+    name VARCHAR(50) NOT NULL,
+    start_date DATE NOT NULL,
+    end_date DATE NOT NULL,
+    status VARCHAR(20) NOT NULL DEFAULT 'open',
+    opening_balance_posted BOOLEAN NOT NULL DEFAULT FALSE,
+    closing_date TIMESTAMPTZ,
+    closed_by VARCHAR(255),
+
+    CONSTRAINT fk_fiscal_year_company
+        FOREIGN KEY (company_id) REFERENCES company_read_models(aggregate_id) ON DELETE CASCADE,
+    CONSTRAINT chk_fiscal_year_status
+        CHECK (status IN ('open', 'closed', 'locked'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_fiscal_year_company ON fiscal_year_read_models(company_id);
+CREATE INDEX IF NOT EXISTS idx_fiscal_year_dates ON fiscal_year_read_models(start_date, end_date);
+
+-- Accounting period read models
+CREATE TABLE IF NOT EXISTS accounting_period_read_models (
+    aggregate_id VARCHAR(255) PRIMARY KEY,
+    create_time TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_time TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    last_aggregate_sequence_number INT NOT NULL DEFAULT 1,
+
+    fiscal_year_id VARCHAR(255) NOT NULL,
+    company_id VARCHAR(255) NOT NULL,
+    name VARCHAR(50) NOT NULL,
+    period_number SMALLINT NOT NULL,
+    start_date DATE NOT NULL,
+    end_date DATE NOT NULL,
+    status VARCHAR(20) NOT NULL DEFAULT 'future',
+    closed_at TIMESTAMPTZ,
+    closed_by VARCHAR(255),
+    reopened_at TIMESTAMPTZ,
+    reopened_by VARCHAR(255),
+    locked_at TIMESTAMPTZ,
+    locked_by VARCHAR(255),
+
+    CONSTRAINT fk_period_fiscal_year
+        FOREIGN KEY (fiscal_year_id) REFERENCES fiscal_year_read_models(aggregate_id) ON DELETE CASCADE,
+    CONSTRAINT chk_period_status
+        CHECK (status IN ('future', 'open', 'closed', 'locked'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_period_fiscal_year ON accounting_period_read_models(fiscal_year_id);
+CREATE INDEX IF NOT EXISTS idx_period_company ON accounting_period_read_models(company_id);
+
+-- Account read models (chart of accounts)
+CREATE TABLE IF NOT EXISTS account_read_models (
+    aggregate_id VARCHAR(255) PRIMARY KEY,
+    create_time TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_time TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    last_aggregate_sequence_number INT NOT NULL DEFAULT 1,
+
+    company_id VARCHAR(255) NOT NULL,
+    account_number VARCHAR(10) NOT NULL,
+    name VARCHAR(255) NOT NULL,
+    account_type VARCHAR(20) NOT NULL,
+    parent_id VARCHAR(255),
+    description TEXT,
+    vat_code_id VARCHAR(255),
+    is_active BOOLEAN NOT NULL DEFAULT TRUE,
+    is_system_account BOOLEAN NOT NULL DEFAULT FALSE,
+
+    CONSTRAINT fk_account_company
+        FOREIGN KEY (company_id) REFERENCES company_read_models(aggregate_id) ON DELETE CASCADE,
+    CONSTRAINT chk_account_type
+        CHECK (account_type IN ('asset', 'liability', 'equity', 'revenue', 'cogs', 'expense', 'personnel', 'financial', 'extraordinary'))
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_account_number
+    ON account_read_models(company_id, account_number);
+CREATE INDEX IF NOT EXISTS idx_account_company ON account_read_models(company_id);
+CREATE INDEX IF NOT EXISTS idx_account_type ON account_read_models(account_type);
--- a/backend/Books.Api/Domain/Companies/CompanyAggregate.cs
+++ b/backend/Books.Api/Domain/Companies/CompanyAggregate.cs
@ -0,0 +1,81 @@
+using Books.Api.Domain.Companies.Events;
+using EventFlow.Aggregates;
+
+namespace Books.Api.Domain.Companies;
+
+public class CompanyAggregate(CompanyId id) : AggregateRoot<CompanyAggregate, CompanyId>(id)
+{
+    private bool _isCreated;
+
+    public void Apply(CompanyCreatedEvent e) => _isCreated = true;
+
+    public void Apply(CompanyUpdatedEvent e) { }
+
+    public void Create(
+        string name,
+        string? cvr,
+        string? address,
+        string? postalCode,
+        string? city,
+        string country,
+        int fiscalYearStartMonth,
+        string currency,
+        bool vatRegistered,
+        string? vatPeriodFrequency)
+    {
+        if (_isCreated)
+            throw new DomainException("Company already exists");
+
+        if (string.IsNullOrWhiteSpace(name))
+            throw new DomainException("Company name is required");
+
+        if (fiscalYearStartMonth < 1 || fiscalYearStartMonth > 12)
+            throw new DomainException("Fiscal year start month must be between 1 and 12");
+
+        Emit(new CompanyCreatedEvent(
+            name.Trim(),
+            cvr?.Trim(),
+            address?.Trim(),
+            postalCode?.Trim(),
+            city?.Trim(),
+            country,
+            fiscalYearStartMonth,
+            currency,
+            vatRegistered,
+            vatPeriodFrequency));
+    }
+
+    public void Update(
+        string name,
+        string? cvr,
+        string? address,
+        string? postalCode,
+        string? city,
+        string country,
+        int fiscalYearStartMonth,
+        string currency,
+        bool vatRegistered,
+        string? vatPeriodFrequency)
+    {
+        if (!_isCreated)
+            throw new DomainException("Company does not exist");
+
+        if (string.IsNullOrWhiteSpace(name))
+            throw new DomainException("Company name is required");
+
+        if (fiscalYearStartMonth < 1 || fiscalYearStartMonth > 12)
+            throw new DomainException("Fiscal year start month must be between 1 and 12");
+
+        Emit(new CompanyUpdatedEvent(
+            name.Trim(),
+            cvr?.Trim(),
+            address?.Trim(),
+            postalCode?.Trim(),
+            city?.Trim(),
+            country,
+            fiscalYearStartMonth,
+            currency,
+            vatRegistered,
+            vatPeriodFrequency));
+    }
+}
--- a/backend/Books.Api/Domain/Companies/CompanyId.cs
+++ b/backend/Books.Api/Domain/Companies/CompanyId.cs
@ -0,0 +1,8 @@
+using EventFlow.Core;
+
+namespace Books.Api.Domain.Companies;
+
+public class CompanyId : Identity<CompanyId>
+{
+    public CompanyId(string value) : base(value) { }
+}
--- a/backend/Books.Api/Domain/Companies/Events/CompanyCreatedEvent.cs
+++ b/backend/Books.Api/Domain/Companies/Events/CompanyCreatedEvent.cs
@ -0,0 +1,27 @@
+using EventFlow.Aggregates;
+
+namespace Books.Api.Domain.Companies.Events;
+
+public class CompanyCreatedEvent(
+    string name,
+    string? cvr,
+    string? address,
+    string? postalCode,
+    string? city,
+    string country,
+    int fiscalYearStartMonth,
+    string currency,
+    bool vatRegistered,
+    string? vatPeriodFrequency) : AggregateEvent<CompanyAggregate, CompanyId>
+{
+    public string Name { get; } = name;
+    public string? Cvr { get; } = cvr;
+    public string? Address { get; } = address;
+    public string? PostalCode { get; } = postalCode;
+    public string? City { get; } = city;
+    public string Country { get; } = country;
+    public int FiscalYearStartMonth { get; } = fiscalYearStartMonth;
+    public string Currency { get; } = currency;
+    public bool VatRegistered { get; } = vatRegistered;
+    public string? VatPeriodFrequency { get; } = vatPeriodFrequency;
+}
--- a/backend/Books.Api/Domain/Companies/Events/CompanyUpdatedEvent.cs
+++ b/backend/Books.Api/Domain/Companies/Events/CompanyUpdatedEvent.cs
@ -0,0 +1,27 @@
+using EventFlow.Aggregates;
+
+namespace Books.Api.Domain.Companies.Events;
+
+public class CompanyUpdatedEvent(
+    string name,
+    string? cvr,
+    string? address,
+    string? postalCode,
+    string? city,
+    string country,
+    int fiscalYearStartMonth,
+    string currency,
+    bool vatRegistered,
+    string? vatPeriodFrequency) : AggregateEvent<CompanyAggregate, CompanyId>
+{
+    public string Name { get; } = name;
+    public string? Cvr { get; } = cvr;
+    public string? Address { get; } = address;
+    public string? PostalCode { get; } = postalCode;
+    public string? City { get; } = city;
+    public string Country { get; } = country;
+    public int FiscalYearStartMonth { get; } = fiscalYearStartMonth;
+    public string Currency { get; } = currency;
+    public bool VatRegistered { get; } = vatRegistered;
+    public string? VatPeriodFrequency { get; } = vatPeriodFrequency;
+}
--- a/backend/Books.Api/Domain/DomainException.cs
+++ b/backend/Books.Api/Domain/DomainException.cs
@ -0,0 +1,20 @@
+namespace Books.Api.Domain;
+
+public class DomainException : Exception
+{
+    public string Code { get; }
+    public string? MessageDanish { get; }
+
+    public DomainException(string message)
+        : base(message)
+    {
+        Code = "DOMAIN_ERROR";
+    }
+
+    public DomainException(string code, string message, string messageDanish)
+        : base(message)
+    {
+        Code = code;
+        MessageDanish = messageDanish;
+    }
+}
--- a/backend/Books.Api/Endpoints/ApiEndpoints.cs
+++ b/backend/Books.Api/Endpoints/ApiEndpoints.cs
--- a/backend/Books.Api/EventFlow/Customs/Casing.cs
+++ b/backend/Books.Api/EventFlow/Customs/Casing.cs
@ -0,0 +1,31 @@
+using System.Reflection;
+
+namespace Books.Api.EventFlow.Customs;
+
+public static class Casing
+{
+    public static string ToSnakeCase(this string input)
+    {
+        if (string.IsNullOrEmpty(input))
+            return input;
+
+        var stringBuilder = new System.Text.StringBuilder();
+        for (var i = 0; i < input.Length; i++)
+        {
+            var c = input[i];
+            if (char.IsUpper(c))
+            {
+                if (i > 0)
+                {
+                    stringBuilder.Append('_');
+                }
+                stringBuilder.Append(char.ToLower(c));
+            }
+            else
+            {
+                stringBuilder.Append(c);
+            }
+        }
+        return stringBuilder.ToString();
+    }
+}
--- a/backend/Books.Api/EventFlow/Customs/ReadModelSqlGenerator.cs
+++ b/backend/Books.Api/EventFlow/Customs/ReadModelSqlGenerator.cs
@ -0,0 +1,221 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2015-2025 Rasmus Mikkelsen
+// https://github.com/eventflow/EventFlow
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of
+// this software and associated documentation files (the "Software"), to deal in
+// the Software without restriction, including without limitation the rights to
+// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+// the Software, and to permit persons to whom the Software is furnished to do so,
+// subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+// FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+// COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+// IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+// CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+using System.Collections.Concurrent;
+using System.ComponentModel.DataAnnotations.Schema;
+using System.Reflection;
+using EventFlow.Extensions;
+using EventFlow.ReadStores;
+using EventFlow.Sql.ReadModels;
+using EventFlow.Sql.ReadModels.Attributes;
+
+namespace Books.Api.EventFlow.Customs;
+
+/// <summary>
+/// Custom SQL generator for PostgreSQL that uses snake_case column names
+/// while keeping PascalCase property names for Dapper parameter binding.
+/// </summary>
+public class ReadModelSqlGenerator : IReadModelSqlGenerator
+{
+    private static readonly ConcurrentDictionary<Type, string> TableNames = new();
+    private static readonly ConcurrentDictionary<Type, IReadOnlyCollection<PropertyInfo>> PropertyInfos = new();
+    private static readonly ConcurrentDictionary<Type, string> IdentityColumns = new();
+    private static readonly ConcurrentDictionary<Type, string> IdentityProperties = new();
+    private static readonly ConcurrentDictionary<Type, string> VersionColumns = new();
+
+    private readonly ConcurrentDictionary<Type, string?> _insertSqls = new();
+    private readonly ConcurrentDictionary<Type, string> _purgeSqls = new();
+    private readonly ConcurrentDictionary<Type, string> _deleteSqls = new();
+    private readonly ConcurrentDictionary<Type, string> _selectSqls = new();
+    private readonly ConcurrentDictionary<Type, string> _updateSqls = new();
+
+    public string? CreateInsertSql<TReadModel>() where TReadModel : IReadModel
+    {
+        var readModelType = typeof(TReadModel);
+        if (_insertSqls.TryGetValue(readModelType, out var sql))
+        {
+            return sql;
+        }
+
+        var properties = GetPropertyInfos(readModelType);
+
+        // Column names in snake_case
+        var columnList = string.Join(", ", properties.Select(p => p.Name.ToSnakeCase()));
+
+        // Parameter names use property names (PascalCase) for Dapper binding
+        var parameterList = string.Join(", ", properties.Select(p => $"@{p.Name}"));
+
+        sql = $"INSERT INTO {GetTableName<TReadModel>()} ({columnList}) VALUES ({parameterList})";
+
+        _insertSqls[readModelType] = sql;
+
+        return sql;
+    }
+
+    public string CreateSelectSql<TReadModel>() where TReadModel : IReadModel
+    {
+        var readModelType = typeof(TReadModel);
+        if (_selectSqls.TryGetValue(readModelType, out var sql))
+        {
+            return sql;
+        }
+
+        var tableName = GetTableName<TReadModel>();
+        var identityColumn = GetIdentityColumn<TReadModel>();
+        var properties = GetPropertyInfos(readModelType);
+
+        // Use explicit column aliases to map snake_case columns to PascalCase properties
+        var columnList = string.Join(", ", properties.Select(p => $"{p.Name.ToSnakeCase()} AS \"{p.Name}\""));
+
+        sql = $"SELECT {columnList} FROM {tableName} WHERE {identityColumn} = @EventFlowReadModelId";
+
+        _selectSqls[readModelType] = sql;
+
+        return sql;
+    }
+
+    public string CreateDeleteSql<TReadModel>() where TReadModel : IReadModel
+    {
+        var readModelType = typeof(TReadModel);
+        if (_deleteSqls.TryGetValue(readModelType, out var sql))
+        {
+            return sql;
+        }
+
+        sql = $"DELETE FROM {GetTableName<TReadModel>()} WHERE {GetIdentityColumn<TReadModel>()} = @EventFlowReadModelId";
+        _deleteSqls[readModelType] = sql;
+
+        return sql;
+    }
+
+    public string CreateUpdateSql<TReadModel>() where TReadModel : IReadModel
+    {
+        var readModelType = typeof(TReadModel);
+        if (_updateSqls.TryGetValue(readModelType, out var sql))
+        {
+            return sql;
+        }
+
+        var identityColumn = GetIdentityColumn<TReadModel>();
+        var identityProperty = GetIdentityProperty<TReadModel>();
+        var versionColumn = GetVersionColumn<TReadModel>();
+
+        var versionCheck = string.IsNullOrEmpty(versionColumn)
+            ? string.Empty
+            : $"AND {versionColumn} = @_PREVIOUS_VERSION";
+
+        var properties = GetPropertyInfos(readModelType);
+        var updateColumns = properties
+            .Where(p => p.Name.ToSnakeCase() != identityColumn)
+            .Select(p => $"{p.Name.ToSnakeCase()} = @{p.Name}");
+
+        var tableName = GetTableName<TReadModel>();
+
+        sql = $"UPDATE {tableName} SET {string.Join(", ", updateColumns)} WHERE {identityColumn} = @{identityProperty} {versionCheck}";
+
+        _updateSqls[readModelType] = sql;
+
+        return sql;
+    }
+
+    public string CreatePurgeSql<TReadModel>() where TReadModel : IReadModel
+    {
+        return _purgeSqls.GetOrCreate(typeof(TReadModel), t => $"DELETE FROM {GetTableName(t)}");
+    }
+
+    public string GetTableName<TReadModel>() where TReadModel : IReadModel
+    {
+        return GetTableName(typeof(TReadModel));
+    }
+
+    private static string GetTableName(Type readModelType)
+    {
+        return TableNames.GetOrAdd(
+            readModelType,
+            t =>
+            {
+                var tableAttribute = t.GetTypeInfo().GetCustomAttribute<TableAttribute>(false);
+                var table = string.IsNullOrEmpty(tableAttribute?.Name)
+                    ? $"ReadModel-{t.Name.Replace("ReadModel", string.Empty)}"
+                    : tableAttribute.Name;
+                return string.IsNullOrEmpty(tableAttribute?.Schema)
+                    ? $"\"{table}\""
+                    : $"\"{tableAttribute?.Schema}\".\"{table}\"";
+            });
+    }
+
+    private static string GetIdentityColumn<TReadModel>()
+    {
+        return IdentityColumns.GetOrAdd(
+            typeof(TReadModel),
+            t =>
+            {
+                var propertyInfo = GetPropertyInfos(t)
+                    .SingleOrDefault(
+                        pi => pi.GetCustomAttributes().Any(a => a is SqlReadModelIdentityColumnAttribute));
+                return (propertyInfo?.Name ?? "AggregateId").ToSnakeCase();
+            });
+    }
+
+    private static string GetIdentityProperty<TReadModel>()
+    {
+        return IdentityProperties.GetOrAdd(
+            typeof(TReadModel),
+            t =>
+            {
+                var propertyInfo = GetPropertyInfos(t)
+                    .SingleOrDefault(
+                        pi => pi.GetCustomAttributes().Any(a => a is SqlReadModelIdentityColumnAttribute));
+                return propertyInfo?.Name ?? "AggregateId";
+            });
+    }
+
+    private static string GetVersionColumn<TReadModel>()
+    {
+        return VersionColumns.GetOrAdd(
+            typeof(TReadModel),
+            t =>
+            {
+                var propertyInfo = GetPropertyInfos(t)
+                    .SingleOrDefault(
+                        pi => pi.GetCustomAttributes().Any(a => a is SqlReadModelVersionColumnAttribute));
+                if (propertyInfo != null)
+                {
+                    return propertyInfo.Name.ToSnakeCase();
+                }
+
+                return GetPropertyInfos(t).Any(n => n.Name == "LastAggregateSequenceNumber")
+                    ? "last_aggregate_sequence_number"
+                    : string.Empty;
+            });
+    }
+
+    private static IReadOnlyCollection<PropertyInfo> GetPropertyInfos(Type readModelType)
+    {
+        return PropertyInfos.GetOrAdd(
+            readModelType,
+            t => t.GetProperties(BindingFlags.Instance | BindingFlags.Public)
+                .Where(p => !p.GetCustomAttributes().Any(a => a is SqlReadModelIgnoreColumnAttribute))
+                .OrderBy(p => p.Name)
+                .ToList());
+    }
+}
--- a/backend/Books.Api/EventFlow/Extensions/ReadModelRegistrationExtensions.cs
+++ b/backend/Books.Api/EventFlow/Extensions/ReadModelRegistrationExtensions.cs
@ -0,0 +1,30 @@
+using Books.Api.EventFlow.ReadModels;
+using Books.Api.EventFlow.Repositories;
+using EventFlow;
+using EventFlow.Extensions;
+using EventFlow.PostgreSql.Extensions;
+using EventFlow.Sql.ReadModels;
+using ReadModelSqlGenerator = Books.Api.EventFlow.Customs.ReadModelSqlGenerator;
+
+namespace Books.Api.EventFlow.Extensions;
+
+public static class ReadModelRegistrationExtensions
+{
+    public static IEventFlowOptions AddReadModels(this IEventFlowOptions options)
+    {
+        return options
+            .UsePostgreSqlReadModel<CompanyReadModel, CompanyReadModelLocator>()
+            .RegisterServices( sr => sr.AddSingleton<IReadModelSqlGenerator>(new ReadModelSqlGenerator()));
+    }
+
+    public static IServiceCollection AddRepositories(this IServiceCollection services)
+    {
+        // Register locators
+        services.AddTransient<CompanyReadModelLocator>();
+
+        // Register repositories
+        services.AddScoped<ICompanyRepository, CompanyRepository>();
+
+        return services;
+    }
+}
--- a/backend/Books.Api/EventFlow/Infrastructure/DatabaseMigrator.cs
+++ b/backend/Books.Api/EventFlow/Infrastructure/DatabaseMigrator.cs
@ -0,0 +1,43 @@
+using System.Reflection;
+using Dapper;
+using DbUp;
+using Npgsql;
+
+namespace Books.Api.EventFlow.Infrastructure;
+
+public static class DatabaseMigrator
+{
+    public static void Migrate(string connectionString)
+    {
+        EnsureDatabaseExists(connectionString);
+
+        var upgrader = DeployChanges.To
+            .PostgresqlDatabase(connectionString)
+            .WithScriptsEmbeddedInAssembly(Assembly.GetExecutingAssembly())
+            .LogToConsole()
+            .Build();
+
+        var result = upgrader.PerformUpgrade();
+        if (!result.Successful)
+            throw result.Error;
+    }
+
+    private static void EnsureDatabaseExists(string connectionString)
+    {
+        var builder = new NpgsqlConnectionStringBuilder(connectionString);
+        var database = builder.Database;
+        builder.Database = "postgres";
+
+        using var connection = new NpgsqlConnection(builder.ConnectionString);
+        connection.Open();
+
+        var exists = connection.ExecuteScalar<bool>(
+            "SELECT EXISTS(SELECT 1 FROM pg_database WHERE datname = @db)",
+            new { db = database });
+
+        if (!exists)
+        {
+            connection.Execute($"CREATE DATABASE \"{database}\"");
+        }
+    }
+}
--- a/backend/Books.Api/EventFlow/Infrastructure/DispatchToSubscriberResilienceStrategy.cs
+++ b/backend/Books.Api/EventFlow/Infrastructure/DispatchToSubscriberResilienceStrategy.cs
@ -0,0 +1,238 @@
+using System.Reflection;
+using Books.Api.Infrastructure;
+using EventFlow.Aggregates;
+using EventFlow.EventStores;
+using EventFlow.Subscribers;
+using Hangfire;
+
+namespace Books.Api.EventFlow.Infrastructure;
+
+#pragma warning disable CS9113 // Parameter is unread
+public class DispatchToSubscriberResilienceStrategy(
+    ILogger<DispatchToSubscriberResilienceStrategy> logger,
+    IScheduler scheduler,
+    IServiceProvider serviceProvider,
+    IEventJsonSerializer eventJsonSerializer) : IDispatchToSubscriberResilienceStrategy
+#pragma warning restore CS9113
+{
+    public Task BeforeHandleEventAsync(
+        ISubscribe subscriberTo,
+        IDomainEvent domainEvent,
+        CancellationToken cancellationToken)
+    {
+        return Task.CompletedTask;
+    }
+
+    public Task HandleEventFailedAsync(
+        ISubscribe subscriberTo,
+        IDomainEvent domainEvent,
+        Exception exception,
+        bool swallowException,
+        CancellationToken cancellationToken)
+    {
+        var subscriberType = GetSubscriberType(subscriberTo);
+        var eventType = domainEvent.EventType.Name;
+        var aggregateId = domainEvent.GetIdentity()?.Value ?? "unknown";
+
+        logger.LogError(exception,
+            "[RESILIENCE] Subscriber {SubscriberType} failed to handle event {EventType} for aggregate {AggregateId}",
+            subscriberType.Name,
+            eventType,
+            aggregateId);
+
+        try
+        {
+            var serializedEvent = eventJsonSerializer.Serialize(domainEvent);
+            var domainEventInterface = domainEvent.GetType().GetTypeInfo()
+                .GetInterfaces()
+                .FirstOrDefault(i => i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IDomainEvent<,,>));
+
+            if (domainEventInterface != null)
+            {
+                scheduler.EnqueueJob<SubscriberRetryJob>(
+                    job => job.RetryEventDispatchAsync(
+                        subscriberType.AssemblyQualifiedName!,
+                        domainEventInterface.AssemblyQualifiedName!,
+                        serializedEvent.SerializedData,
+                        serializedEvent.SerializedMetadata),
+                    TimeSpan.FromSeconds(30));
+
+                logger.LogWarning(
+                    "[RESILIENCE] Scheduled retry job for {SubscriberType} / {EventType} / {AggregateId}",
+                    subscriberType.Name,
+                    eventType,
+                    aggregateId);
+            }
+        }
+        catch (Exception e)
+        {
+            logger.LogError(e,
+                "[RESILIENCE] Failed to schedule retry for {SubscriberType} / {EventType}. " +
+                "This event will NOT be retried and may cause data inconsistency.",
+                subscriberType.Name,
+                eventType);
+        }
+
+        return Task.CompletedTask;
+    }
+
+    public Task HandleEventSucceededAsync(
+        ISubscribe subscriberTo,
+        IDomainEvent domainEvent,
+        CancellationToken cancellationToken)
+    {
+        return Task.CompletedTask;
+    }
+
+    public Task BeforeDispatchToSubscribersAsync(
+        IDomainEvent domainEvent,
+        IReadOnlyCollection<IDomainEvent> domainEvents,
+        CancellationToken cancellationToken)
+    {
+        return Task.CompletedTask;
+    }
+
+    public Task DispatchToSubscribersSucceededAsync(
+        IDomainEvent domainEvent,
+        IReadOnlyCollection<IDomainEvent> domainEvents,
+        CancellationToken cancellationToken)
+    {
+        return Task.CompletedTask;
+    }
+
+    public Task<bool> HandleDispatchToSubscribersFailedAsync(
+        IDomainEvent domainEvent,
+        IReadOnlyCollection<IDomainEvent> domainEvents,
+        Exception exception,
+        CancellationToken cancellationToken)
+    {
+        return Task.FromResult(true);
+    }
+
+    private static Type GetSubscriberType(ISubscribe subscriberTo)
+    {
+        if (subscriberTo is ISubscribeDecorator decorator)
+        {
+            return GetSubscriberType(decorator.InnerInstance as ISubscribe
+                ?? throw new InvalidOperationException("InnerInstance is not ISubscribe"));
+        }
+
+        return subscriberTo.GetType();
+    }
+}
+
+public interface ISubscribeDecorator
+{
+    object InnerInstance { get; }
+}
+
+public class SubscriberRetryJob(
+    ILogger<SubscriberRetryJob> logger,
+    IServiceProvider serviceProvider,
+    IEventJsonSerializer eventJsonSerializer)
+{
+    [AutomaticRetry(Attempts = 3, DelaysInSeconds = [30, 60, 120])]
+    public async Task RetryEventDispatchAsync(
+        string subscriberTypeName,
+        string domainEventTypeName,
+        string serializedData,
+        string serializedMetadata)
+    {
+        logger.LogInformation(
+            "[RETRY] Retrying event dispatch to {SubscriberType}",
+            subscriberTypeName);
+        try
+        {
+            var domainEvent = eventJsonSerializer.Deserialize(serializedData, serializedMetadata);
+            var subscriberType = Type.GetType(subscriberTypeName);
+
+            if (subscriberType == null)
+            {
+                logger.LogError("[RETRY] Could not resolve subscriber type: {SubscriberType}", subscriberTypeName);
+                return;
+            }
+
+            var subscriber = FindSubscriberInstance(subscriberType, domainEvent);
+            if (subscriber == null)
+            {
+                logger.LogError("[RETRY] Could not find subscriber instance for {SubscriberType}", subscriberTypeName);
+                return;
+            }
+
+            var domainEventType = domainEvent.GetType();
+            var handleAsyncMethod = subscriber.GetType().GetMethods()
+                .FirstOrDefault(m => m.Name == "HandleAsync" &&
+                    m.GetParameters().Any(p => p.ParameterType.IsAssignableFrom(domainEventType)));
+
+            if (handleAsyncMethod != null)
+            {
+                await (Task)handleAsyncMethod.Invoke(subscriber, [domainEvent, CancellationToken.None])!;
+                logger.LogInformation("[RETRY] Successfully retried event dispatch to {SubscriberType}", subscriberTypeName);
+            }
+            else
+            {
+                logger.LogError(
+                    "[RETRY] No matching HandleAsync method found for {SubscriberType} and {DomainEventType}",
+                    subscriberTypeName,
+                    domainEventTypeName);
+            }
+        }
+        catch (Exception ex)
+        {
+            logger.LogError(ex, "[RETRY] Failed to retry event dispatch to {SubscriberType}", subscriberTypeName);
+            throw;
+        }
+    }
+
+    private object? FindSubscriberInstance(Type subscriberType, object domainEvent)
+    {
+        var interfaceType = ExtractSubscriberInterface(subscriberType, domainEvent);
+        if (interfaceType == null)
+            return null;
+
+        var instances = serviceProvider.GetServices(interfaceType);
+        return instances.FirstOrDefault(x =>
+            x?.GetType() == subscriberType ||
+            HasInnerType(x, subscriberType));
+    }
+
+    private static Type? ExtractSubscriberInterface(Type subscriberType, object domainEvent)
+    {
+        var domainEventType = domainEvent.GetType();
+        var interfaces = subscriberType.GetInterfaces()
+            .Where(i => i.IsGenericType &&
+                (i.GetGenericTypeDefinition() == typeof(ISubscribeAsynchronousTo<,,>) ||
+                 i.GetGenericTypeDefinition() == typeof(ISubscribeSynchronousTo<,,>)));
+
+        foreach (var iface in interfaces)
+        {
+            var genericArguments = iface.GetGenericArguments();
+            var eventGenericArgs = domainEventType.GenericTypeArguments;
+            if (eventGenericArgs.Length >= 3 && genericArguments[2] == eventGenericArgs[2])
+            {
+                return iface;
+            }
+        }
+
+        return null;
+    }
+
+    private static bool HasInnerType(object? obj, Type concreteType)
+    {
+        if (obj == null) return false;
+
+        var decoratorType = obj.GetType();
+        var field = decoratorType.GetFields(BindingFlags.NonPublic | BindingFlags.Instance)
+            .FirstOrDefault(x => x.Name.Contains("inner", StringComparison.InvariantCultureIgnoreCase));
+        var property = decoratorType.GetProperties(BindingFlags.NonPublic | BindingFlags.Instance)
+            .FirstOrDefault(x => x.Name.Contains("inner", StringComparison.InvariantCultureIgnoreCase));
+
+        if (field?.GetValue(obj)?.GetType() == concreteType)
+            return true;
+
+        if (property?.GetValue(obj)?.GetType() == concreteType)
+            return true;
+
+        return false;
+    }
+}
--- a/backend/Books.Api/EventFlow/Infrastructure/ReadStoresResilienceStrategy.cs
+++ b/backend/Books.Api/EventFlow/Infrastructure/ReadStoresResilienceStrategy.cs
@ -0,0 +1,95 @@
+using Books.Api.Infrastructure;
+using EventFlow.Aggregates;
+using EventFlow.ReadStores;
+using Hangfire;
+
+namespace Books.Api.EventFlow.Infrastructure;
+
+public class ReadStoresResilienceStrategy(
+    ILogger<ReadStoresResilienceStrategy> logger,
+    IScheduler scheduler) : IDispatchToReadStoresResilienceStrategy
+{
+    public Task BeforeUpdateAsync(
+        IReadStoreManager readStoreManager,
+        IReadOnlyCollection<IDomainEvent> domainEvents,
+        CancellationToken cancellationToken)
+    {
+        return Task.CompletedTask;
+    }
+
+    public Task<bool> HandleUpdateFailedAsync(
+        IReadStoreManager readStoreManager,
+        IReadOnlyCollection<IDomainEvent> domainEvents,
+        Exception exception,
+        CancellationToken cancellationToken)
+    {
+        var firstEvent = domainEvents.FirstOrDefault();
+        var aggregateId = firstEvent?.GetIdentity()?.Value ?? "unknown";
+        var readModelType = readStoreManager.ReadModelType.Name;
+
+        logger.LogError(exception,
+            "[RESILIENCE] Failed to update read model {ReadModelType} for aggregate {AggregateId}. " +
+            "Events: {EventCount}. This may cause data inconsistency.",
+            readModelType,
+            aggregateId,
+            domainEvents.Count);
+
+        // Schedule a retry job to repopulate the read model
+        if (firstEvent != null)
+        {
+            var aggregateType = firstEvent.AggregateType.Name;
+
+            scheduler.EnqueueJob<ReadModelRepopulationJob>(
+                job => job.RepopulateReadModelAsync(
+                    aggregateType,
+                    aggregateId,
+                    readModelType),
+                TimeSpan.FromSeconds(30));
+
+            logger.LogWarning(
+                "[RESILIENCE] Scheduled read model repopulation job for {ReadModelType} / {AggregateId}",
+                readModelType,
+                aggregateId);
+        }
+
+        // Return false to indicate we handled the failure (don't rethrow)
+        return Task.FromResult(false);
+    }
+
+    public Task UpdateSucceededAsync(
+        IReadStoreManager readStoreManager,
+        IReadOnlyCollection<IDomainEvent> domainEvents,
+        CancellationToken cancellationToken)
+    {
+        return Task.CompletedTask;
+    }
+}
+
+public class ReadModelRepopulationJob(ILogger<ReadModelRepopulationJob> logger)
+{
+    [AutomaticRetry(Attempts = 3, DelaysInSeconds = [30, 60, 120])]
+    public Task RepopulateReadModelAsync(
+        string aggregateType,
+        string aggregateId,
+        string readModelType)
+    {
+        logger.LogInformation(
+            "[REPOPULATION] Starting read model repopulation for {ReadModelType} / {AggregateId}",
+            readModelType,
+            aggregateId);
+
+        // For now, we log the repopulation attempt
+        // A full implementation would:
+        // 1. Load all events for the aggregate from the event store
+        // 2. Clear the existing read model entry
+        // 3. Replay all events to rebuild the read model
+
+        logger.LogWarning(
+            "[REPOPULATION] Read model repopulation for {ReadModelType} / {AggregateId} " +
+            "requires manual intervention. Check data consistency.",
+            readModelType,
+            aggregateId);
+
+        return Task.CompletedTask;
+    }
+}
--- a/backend/Books.Api/EventFlow/ReadModels/CompanyReadModel.cs
+++ b/backend/Books.Api/EventFlow/ReadModels/CompanyReadModel.cs
@ -0,0 +1,80 @@
+using System.ComponentModel.DataAnnotations.Schema;
+using Books.Api.Domain.Companies;
+using Books.Api.Domain.Companies.Events;
+using EventFlow.Aggregates;
+using EventFlow.PostgreSql.ReadStores.Attributes;
+using EventFlow.ReadStores;
+
+namespace Books.Api.EventFlow.ReadModels;
+
+[Table("company_read_models")]
+public class CompanyReadModel : IReadModel,
+    IAmReadModelFor<CompanyAggregate, CompanyId, CompanyCreatedEvent>,
+    IAmReadModelFor<CompanyAggregate, CompanyId, CompanyUpdatedEvent>
+{
+    // EventFlow standard columns
+    [PostgreSqlReadModelIdentityColumn]
+    public string AggregateId { get; set; } = string.Empty;
+
+    public DateTimeOffset CreateTime { get; set; }
+    public DateTimeOffset UpdatedTime { get; set; }
+
+    [PostgreSqlReadModelVersionColumn]
+    public int LastAggregateSequenceNumber { get; set; }
+
+    // Business columns
+    public string Name { get; set; } = string.Empty;
+    public string? Cvr { get; set; }
+    public string? Address { get; set; }
+    public string? PostalCode { get; set; }
+    public string? City { get; set; }
+    public string Country { get; set; } = "DK";
+    public int FiscalYearStartMonth { get; set; } = 1;
+    public string Currency { get; set; } = "DKK";
+    public bool VatRegistered { get; set; }
+    public string? VatPeriodFrequency { get; set; }
+
+    public Task ApplyAsync(
+        IReadModelContext context,
+        IDomainEvent<CompanyAggregate, CompanyId, CompanyCreatedEvent> domainEvent,
+        CancellationToken cancellationToken)
+    {
+        var e = domainEvent.AggregateEvent;
+        AggregateId = domainEvent.AggregateIdentity.Value;
+        CreateTime = domainEvent.Timestamp;
+        UpdatedTime = domainEvent.Timestamp;
+        LastAggregateSequenceNumber = (int)domainEvent.AggregateSequenceNumber;
+        Name = e.Name;
+        Cvr = e.Cvr;
+        Address = e.Address;
+        PostalCode = e.PostalCode;
+        City = e.City;
+        Country = e.Country;
+        FiscalYearStartMonth = e.FiscalYearStartMonth;
+        Currency = e.Currency;
+        VatRegistered = e.VatRegistered;
+        VatPeriodFrequency = e.VatPeriodFrequency;
+        return Task.CompletedTask;
+    }
+
+    public Task ApplyAsync(
+        IReadModelContext context,
+        IDomainEvent<CompanyAggregate, CompanyId, CompanyUpdatedEvent> domainEvent,
+        CancellationToken cancellationToken)
+    {
+        var e = domainEvent.AggregateEvent;
+        UpdatedTime = domainEvent.Timestamp;
+        LastAggregateSequenceNumber = (int)domainEvent.AggregateSequenceNumber;
+        Name = e.Name;
+        Cvr = e.Cvr;
+        Address = e.Address;
+        PostalCode = e.PostalCode;
+        City = e.City;
+        Country = e.Country;
+        FiscalYearStartMonth = e.FiscalYearStartMonth;
+        Currency = e.Currency;
+        VatRegistered = e.VatRegistered;
+        VatPeriodFrequency = e.VatPeriodFrequency;
+        return Task.CompletedTask;
+    }
+}
--- a/backend/Books.Api/EventFlow/ReadModels/CompanyReadModelDto.cs
+++ b/backend/Books.Api/EventFlow/ReadModels/CompanyReadModelDto.cs
@ -0,0 +1,24 @@
+namespace Books.Api.EventFlow.ReadModels;
+
+/// <summary>
+/// DTO for reading company data from the database.
+/// Uses a class with properties instead of a positional record because
+/// PostgreSQL returns column names in lowercase, and Dapper matches
+/// properties case-insensitively but requires exact constructor parameter names.
+/// </summary>
+public class CompanyReadModelDto
+{
+    public string Id { get; set; } = string.Empty;
+    public string Name { get; set; } = string.Empty;
+    public string? Cvr { get; set; }
+    public string? Address { get; set; }
+    public string? PostalCode { get; set; }
+    public string? City { get; set; }
+    public string Country { get; set; } = string.Empty;
+    public int FiscalYearStartMonth { get; set; }
+    public string Currency { get; set; } = string.Empty;
+    public bool VatRegistered { get; set; }
+    public string? VatPeriodFrequency { get; set; }
+    public DateTime CreatedAt { get; set; }
+    public DateTime UpdatedAt { get; set; }
+}
--- a/backend/Books.Api/EventFlow/ReadModels/CompanyReadModelLocator.cs
+++ b/backend/Books.Api/EventFlow/ReadModels/CompanyReadModelLocator.cs
@ -0,0 +1,16 @@
+using Books.Api.Domain.Companies;
+using EventFlow.Aggregates;
+using EventFlow.ReadStores;
+
+namespace Books.Api.EventFlow.ReadModels;
+
+public class CompanyReadModelLocator : IReadModelLocator
+{
+    public IEnumerable<string> GetReadModelIds(IDomainEvent domainEvent)
+    {
+        if (domainEvent is IDomainEvent<CompanyAggregate, CompanyId> typedEvent)
+        {
+            yield return typedEvent.AggregateIdentity.Value;
+        }
+    }
+}
--- a/backend/Books.Api/EventFlow/Repositories/CompanyRepository.cs
+++ b/backend/Books.Api/EventFlow/Repositories/CompanyRepository.cs
@ -0,0 +1,89 @@
+using Books.Api.Domain.Companies;
+using Books.Api.EventFlow.ReadModels;
+using Dapper;
+using Npgsql;
+
+namespace Books.Api.EventFlow.Repositories;
+
+public class CompanyRepository(NpgsqlDataSource dataSource) : ICompanyRepository
+{
+    public async Task<CompanyReadModelDto?> GetByIdAsync(string id, CancellationToken cancellationToken = default)
+    {
+        await using var connection = await dataSource.OpenConnectionAsync(cancellationToken);
+
+        const string sql = """
+            SELECT
+                aggregate_id AS Id,
+                name AS Name,
+                cvr AS Cvr,
+                address AS Address,
+                postal_code AS PostalCode,
+                city AS City,
+                country AS Country,
+                fiscal_year_start_month AS FiscalYearStartMonth,
+                currency AS Currency,
+                vat_registered AS VatRegistered,
+                vat_period_frequency AS VatPeriodFrequency,
+                create_time AS CreatedAt,
+                updated_time AS UpdatedAt
+            FROM company_read_models
+            WHERE aggregate_id = @Id
+            """;
+
+        return await connection.QuerySingleOrDefaultAsync<CompanyReadModelDto>(sql, new { Id = id });
+    }
+
+    public async Task<IEnumerable<CompanyReadModelDto>> GetByIds(List<CompanyId> ids,
+        CancellationToken cancellationToken = default)
+    {
+        await using var connection = await dataSource.OpenConnectionAsync(cancellationToken);
+
+        const string sql = """
+            SELECT
+                aggregate_id AS Id,
+                name AS Name,
+                cvr AS Cvr,
+                address AS Address,
+                postal_code AS PostalCode,
+                city AS City,
+                country AS Country,
+                fiscal_year_start_month AS FiscalYearStartMonth,
+                currency AS Currency,
+                vat_registered AS VatRegistered,
+                vat_period_frequency AS VatPeriodFrequency,
+                create_time AS CreatedAt,
+                updated_time AS UpdatedAt
+            FROM company_read_models
+            WHERE aggregate_id = ANY(@Ids)
+            """;
+
+        return await connection.QueryAsync<CompanyReadModelDto>(sql, new { Ids = ids.Select(i => i.Value).ToArray() });
+    }
+
+    public async Task<IReadOnlyList<CompanyReadModelDto>> GetAllAsync(CancellationToken cancellationToken = default)
+    {
+        await using var connection = await dataSource.OpenConnectionAsync(cancellationToken);
+
+        const string sql = """
+            SELECT
+                aggregate_id AS Id,
+                name AS Name,
+                cvr AS Cvr,
+                address AS Address,
+                postal_code AS PostalCode,
+                city AS City,
+                country AS Country,
+                fiscal_year_start_month AS FiscalYearStartMonth,
+                currency AS Currency,
+                vat_registered AS VatRegistered,
+                vat_period_frequency AS VatPeriodFrequency,
+                create_time AS CreatedAt,
+                updated_time AS UpdatedAt
+            FROM company_read_models
+            ORDER BY name
+            """;
+
+        var result = await connection.QueryAsync<CompanyReadModelDto>(sql);
+        return result.ToList();
+    }
+}
--- a/backend/Books.Api/EventFlow/Repositories/ICompanyRepository.cs
+++ b/backend/Books.Api/EventFlow/Repositories/ICompanyRepository.cs
@ -0,0 +1,11 @@
+using Books.Api.Domain.Companies;
+using Books.Api.EventFlow.ReadModels;
+
+namespace Books.Api.EventFlow.Repositories;
+
+public interface ICompanyRepository
+{
+    Task<CompanyReadModelDto?> GetByIdAsync(string id, CancellationToken cancellationToken = default);
+    Task<IEnumerable<CompanyReadModelDto>> GetByIds(List<CompanyId> ids, CancellationToken cancellationToken = default);
+    Task<IReadOnlyList<CompanyReadModelDto>> GetAllAsync(CancellationToken cancellationToken = default);
+}
--- a/backend/Books.Api/GraphQL/BooksSchema.cs
+++ b/backend/Books.Api/GraphQL/BooksSchema.cs
@ -0,0 +1,14 @@
+using Books.Api.GraphQL.Mutations;
+using Books.Api.GraphQL.Queries;
+using GraphQL.Types;
+
+namespace Books.Api.GraphQL;
+
+public class BooksSchema : Schema
+{
+    public BooksSchema(IServiceProvider serviceProvider) : base(serviceProvider)
+    {
+        Query = serviceProvider.GetRequiredService<BooksQuery>();
+        Mutation = serviceProvider.GetRequiredService<BooksMutation>();
+    }
+}
--- a/backend/Books.Api/GraphQL/InputTypes/CreateCompanyInputType.cs
+++ b/backend/Books.Api/GraphQL/InputTypes/CreateCompanyInputType.cs
@ -0,0 +1,37 @@
+using GraphQL.Types;
+
+namespace Books.Api.GraphQL.InputTypes;
+
+public class CreateCompanyInputType : InputObjectGraphType<CreateCompanyInput>
+{
+    public CreateCompanyInputType()
+    {
+        Name = "CreateCompanyInput";
+        Description = "Input for creating a new company";
+
+        Field(x => x.Name).Description("Company name (required)");
+        Field(x => x.Cvr, nullable: true).Description("Danish CVR number");
+        Field(x => x.Address, nullable: true).Description("Street address");
+        Field(x => x.PostalCode, nullable: true).Description("Postal code");
+        Field(x => x.City, nullable: true).Description("City");
+        Field(x => x.Country, nullable: true).Description("Country code (default: DK)");
+        Field(x => x.FiscalYearStartMonth, nullable: true).Description("Month when fiscal year starts (default: 1)");
+        Field(x => x.Currency, nullable: true).Description("Default currency (default: DKK)");
+        Field(x => x.VatRegistered, nullable: true).Description("Whether VAT registered (default: false)");
+        Field(x => x.VatPeriodFrequency, nullable: true).Description("VAT reporting frequency");
+    }
+}
+
+public class CreateCompanyInput
+{
+    public string Name { get; set; } = string.Empty;
+    public string? Cvr { get; set; }
+    public string? Address { get; set; }
+    public string? PostalCode { get; set; }
+    public string? City { get; set; }
+    public string? Country { get; set; }
+    public int? FiscalYearStartMonth { get; set; }
+    public string? Currency { get; set; }
+    public bool? VatRegistered { get; set; }
+    public string? VatPeriodFrequency { get; set; }
+}
--- a/backend/Books.Api/GraphQL/InputTypes/UpdateCompanyInputType.cs
+++ b/backend/Books.Api/GraphQL/InputTypes/UpdateCompanyInputType.cs
@ -0,0 +1,37 @@
+using GraphQL.Types;
+
+namespace Books.Api.GraphQL.InputTypes;
+
+public class UpdateCompanyInputType : InputObjectGraphType<UpdateCompanyInput>
+{
+    public UpdateCompanyInputType()
+    {
+        Name = "UpdateCompanyInput";
+        Description = "Input for updating an existing company";
+
+        Field(x => x.Name).Description("Company name (required)");
+        Field(x => x.Cvr, nullable: true).Description("Danish CVR number");
+        Field(x => x.Address, nullable: true).Description("Street address");
+        Field(x => x.PostalCode, nullable: true).Description("Postal code");
+        Field(x => x.City, nullable: true).Description("City");
+        Field(x => x.Country, nullable: true).Description("Country code");
+        Field(x => x.FiscalYearStartMonth, nullable: true).Description("Month when fiscal year starts");
+        Field(x => x.Currency, nullable: true).Description("Default currency");
+        Field(x => x.VatRegistered, nullable: true).Description("Whether VAT registered");
+        Field(x => x.VatPeriodFrequency, nullable: true).Description("VAT reporting frequency");
+    }
+}
+
+public class UpdateCompanyInput
+{
+    public string Name { get; set; } = string.Empty;
+    public string? Cvr { get; set; }
+    public string? Address { get; set; }
+    public string? PostalCode { get; set; }
+    public string? City { get; set; }
+    public string? Country { get; set; }
+    public int? FiscalYearStartMonth { get; set; }
+    public string? Currency { get; set; }
+    public bool? VatRegistered { get; set; }
+    public string? VatPeriodFrequency { get; set; }
+}
--- a/backend/Books.Api/GraphQL/Mutations/BooksMutation.cs
+++ b/backend/Books.Api/GraphQL/Mutations/BooksMutation.cs
@ -0,0 +1,82 @@
+using Books.Api.Commands.Companies;
+using Books.Api.Domain.Companies;
+using Books.Api.EventFlow.Repositories;
+using Books.Api.GraphQL.InputTypes;
+using Books.Api.GraphQL.Types;
+using EventFlow;
+using GraphQL;
+using GraphQL.Types;
+
+namespace Books.Api.GraphQL.Mutations;
+
+public class BooksMutation : ObjectGraphType
+{
+    public BooksMutation()
+    {
+        Name = "Mutation";
+        Description = "Root mutation for the Books API";
+
+        // createCompany(input: CreateCompanyInput!): CompanyType
+        Field<CompanyType>("createCompany")
+            .Description("Create a new company")
+            .Argument<NonNullGraphType<CreateCompanyInputType>>("input", "The company data")
+            .ResolveAsync(async ctx =>
+            {
+                var input = ctx.GetArgument<CreateCompanyInput>("input");
+                var commandBus = ctx.RequestServices!.GetRequiredService<ICommandBus>();
+                var repository = ctx.RequestServices!.GetRequiredService<ICompanyRepository>();
+
+                var companyId = CompanyId.New;
+
+                var command = new CreateCompanyCommand(
+                    companyId,
+                    input.Name,
+                    input.Cvr,
+                    input.Address,
+                    input.PostalCode,
+                    input.City,
+                    input.Country ?? "DK",
+                    input.FiscalYearStartMonth ?? 1,
+                    input.Currency ?? "DKK",
+                    input.VatRegistered ?? false,
+                    input.VatPeriodFrequency);
+
+                await commandBus.PublishAsync(command, ctx.CancellationToken);
+
+                // Return the created company (eventually consistent)
+                return await repository.GetByIdAsync(companyId.Value, ctx.CancellationToken);
+            });
+
+        // updateCompany(id: ID!, input: UpdateCompanyInput!): CompanyType
+        Field<CompanyType>("updateCompany")
+            .Description("Update an existing company")
+            .Argument<NonNullGraphType<IdGraphType>>("id", "The company ID")
+            .Argument<NonNullGraphType<UpdateCompanyInputType>>("input", "The updated company data")
+            .ResolveAsync(async ctx =>
+            {
+                var id = ctx.GetArgument<string>("id");
+                var input = ctx.GetArgument<UpdateCompanyInput>("input");
+                var commandBus = ctx.RequestServices!.GetRequiredService<ICommandBus>();
+                var repository = ctx.RequestServices!.GetRequiredService<ICompanyRepository>();
+
+                var companyId = CompanyId.With(id);
+
+                var command = new UpdateCompanyCommand(
+                    companyId,
+                    input.Name,
+                    input.Cvr,
+                    input.Address,
+                    input.PostalCode,
+                    input.City,
+                    input.Country ?? "DK",
+                    input.FiscalYearStartMonth ?? 1,
+                    input.Currency ?? "DKK",
+                    input.VatRegistered ?? false,
+                    input.VatPeriodFrequency);
+
+                await commandBus.PublishAsync(command, ctx.CancellationToken);
+
+                return await repository.GetByIdAsync(companyId.Value, ctx.CancellationToken);
+            });
+    }
+}
--- a/backend/Books.Api/GraphQL/Queries/BooksQuery.cs
+++ b/backend/Books.Api/GraphQL/Queries/BooksQuery.cs
@ -0,0 +1,37 @@
+using Books.Api.Domain.Companies;
+using Books.Api.EventFlow.Repositories;
+using Books.Api.GraphQL.Types;
+using GraphQL;
+using GraphQL.Types;
+
+namespace Books.Api.GraphQL.Queries;
+
+public class BooksQuery : ObjectGraphType
+{
+    public BooksQuery()
+    {
+        Name = "Query";
+        Description = "Root query for the Books API";
+
+        // companies: [CompanyType]
+        Field<ListGraphType<CompanyType>>("companies")
+            .Description("Get all companies")
+            .ResolveAsync(async ctx =>
+            {
+                var repository = ctx.RequestServices!.GetRequiredService<ICompanyRepository>();
+                return await repository.GetAllAsync(ctx.CancellationToken);
+            });
+
+        // company(id: ID!): CompanyType
+        Field<CompanyType>("company")
+            .Description("Get a company by ID")
+            .Argument<NonNullGraphType<IdGraphType>>("id", "The company ID")
+            .ResolveAsync(async ctx =>
+            {
+                var id = ctx.GetArgument<string>("id");
+                var repository = ctx.RequestServices!.GetRequiredService<ICompanyRepository>();
+                var companies = await repository.GetByIds([CompanyId.With(id)], ctx.CancellationToken);
+                return companies.FirstOrDefault();
+            });
+    }
+}
--- a/backend/Books.Api/GraphQL/Types/CompanyType.cs
+++ b/backend/Books.Api/GraphQL/Types/CompanyType.cs
@ -0,0 +1,31 @@
+using Books.Api.EventFlow.ReadModels;
+using GraphQL.Types;
+
+namespace Books.Api.GraphQL.Types;
+
+public class CompanyType : ObjectGraphType<CompanyReadModelDto>
+{
+    public CompanyType()
+    {
+        Name = "Company";
+        Description = "A company/organization for bookkeeping";
+
+        Field(x => x.Id).Description("Unique identifier");
+        Field(x => x.Name).Description("Company name");
+        Field(x => x.Cvr, nullable: true).Description("Danish CVR number");
+        Field(x => x.Address, nullable: true).Description("Street address");
+        Field(x => x.PostalCode, nullable: true).Description("Postal code");
+        Field(x => x.City, nullable: true).Description("City");
+        Field(x => x.Country).Description("Country code (e.g., DK)");
+        Field(x => x.FiscalYearStartMonth).Description("Month when fiscal year starts (1-12)");
+        Field(x => x.Currency).Description("Default currency (e.g., DKK)");
+        Field(x => x.VatRegistered).Description("Whether the company is VAT registered");
+        Field(x => x.VatPeriodFrequency, nullable: true).Description("VAT reporting frequency");
+        Field(x => x.CreatedAt).Description("When the company was created");
+        Field(x => x.UpdatedAt).Description("When the company was last updated");
+
+        // Nested fields will be added later:
+        // - fiscalYears: [FiscalYearType] via DataLoader
+        // - accounts: [AccountType] via DataLoader
+    }
+}
--- a/backend/Books.Api/Infrastructure/HangfireScheduler.cs
+++ b/backend/Books.Api/Infrastructure/HangfireScheduler.cs
@ -0,0 +1,27 @@
+using System.Linq.Expressions;
+using Hangfire;
+
+namespace Books.Api.Infrastructure;
+
+public class HangfireScheduler(IBackgroundJobClient jobClient, IRecurringJobManager recurringJobManager) : IScheduler
+{
+    public void EnqueueJob<T>(Expression<Func<T, Task>> methodCall)
+    {
+        jobClient.Enqueue(methodCall);
+    }
+
+    public void EnqueueJob<T>(Expression<Func<T, Task>> methodCall, TimeSpan delay)
+    {
+        jobClient.Schedule(methodCall, delay);
+    }
+
+    public void EnqueueJob(Expression<Func<Task>> methodCall)
+    {
+        jobClient.Enqueue(methodCall);
+    }
+
+    public void AddOrUpdateScheduledJob<T>(string title, Expression<Func<T?, Task>> methodCall, string cron)
+    {
+        recurringJobManager.AddOrUpdate(title, methodCall, cron);
+    }
+}
--- a/backend/Books.Api/Infrastructure/IScheduler.cs
+++ b/backend/Books.Api/Infrastructure/IScheduler.cs
@ -0,0 +1,11 @@
+using System.Linq.Expressions;
+
+namespace Books.Api.Infrastructure;
+
+public interface IScheduler
+{
+    void EnqueueJob<T>(Expression<Func<T, Task>> methodCall);
+    void EnqueueJob<T>(Expression<Func<T, Task>> methodCall, TimeSpan delay);
+    void EnqueueJob(Expression<Func<Task>> methodCall);
+    void AddOrUpdateScheduledJob<T>(string title, Expression<Func<T?, Task>> methodCall, string cron);
+}
--- a/backend/Books.Api/Logging/ServiceCollectionExtensions.cs
+++ b/backend/Books.Api/Logging/ServiceCollectionExtensions.cs
@ -0,0 +1,13 @@
+namespace Books.Api.Logging;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection DecorateAsyncEventHandlersWithLogging(this IServiceCollection services)
+    {
+        // Decoration will be set up once we have event handlers registered
+        // The SubscribeAsynchronousToDecorator wraps handlers with logging
+        // This is a placeholder for now - implement when we have actual event handlers
+
+        return services;
+    }
+}
--- a/backend/Books.Api/Logging/SubscribeAsynchronousToDecorator.cs
+++ b/backend/Books.Api/Logging/SubscribeAsynchronousToDecorator.cs
@ -0,0 +1,48 @@
+using System.Diagnostics;
+using EventFlow.Aggregates;
+using EventFlow.Core;
+using EventFlow.Subscribers;
+
+namespace Books.Api.Logging;
+
+public class SubscribeAsynchronousToDecorator<TAggregate, TIdentity, TEvent>(
+    ISubscribeAsynchronousTo<TAggregate, TIdentity, TEvent> inner,
+    ILogger<SubscribeAsynchronousToDecorator<TAggregate, TIdentity, TEvent>> logger)
+    : ISubscribeAsynchronousTo<TAggregate, TIdentity, TEvent>
+    where TAggregate : IAggregateRoot<TIdentity>
+    where TIdentity : IIdentity
+    where TEvent : IAggregateEvent<TAggregate, TIdentity>
+{
+    public async Task HandleAsync(
+        IDomainEvent<TAggregate, TIdentity, TEvent> domainEvent,
+        CancellationToken cancellationToken)
+    {
+        var eventName = typeof(TEvent).Name;
+        var handlerName = inner.GetType().Name;
+        var aggregateId = domainEvent.AggregateIdentity.Value;
+
+        logger.LogDebug(
+            "Handling {EventName} for {AggregateId} with {HandlerName}",
+            eventName, aggregateId, handlerName);
+
+        var stopwatch = Stopwatch.StartNew();
+
+        try
+        {
+            await inner.HandleAsync(domainEvent, cancellationToken);
+
+            stopwatch.Stop();
+            logger.LogInformation(
+                "Handled {EventName} for {AggregateId} with {HandlerName} in {ElapsedMs}ms",
+                eventName, aggregateId, handlerName, stopwatch.ElapsedMilliseconds);
+        }
+        catch (Exception ex)
+        {
+            stopwatch.Stop();
+            logger.LogError(ex,
+                "Failed to handle {EventName} for {AggregateId} with {HandlerName} after {ElapsedMs}ms",
+                eventName, aggregateId, handlerName, stopwatch.ElapsedMilliseconds);
+            throw;
+        }
+    }
+}
--- a/backend/Books.Api/Program.cs
+++ b/backend/Books.Api/Program.cs
@ -0,0 +1,35 @@
+using Books.Api;
+using Books.Api.GraphQL;
+using GraphQL;
+using GraphQL.Server.Ui.Altair;
+using Hangfire;
+
+// Enable legacy timestamp behavior for Npgsql 6.0+
+// This allows DateTimeOffset with non-UTC offsets to be written to timestamptz columns
+AppContext.SetSwitch("Npgsql.EnableLegacyTimestampBehavior", true);
+
+var builder = WebApplication.CreateBuilder(args);
+
+// Configure EventFlow, Hangfire, GraphQL and all services
+Startup.ConfigureServices(builder.Services, builder.Configuration, builder.Environment);
+
+var app = builder.Build();
+
+// Configure the HTTP request pipeline
+if (app.Environment.IsDevelopment())
+{
+    app.UseHangfireDashboard();
+}
+
+app.UseHttpsRedirection();
+
+// GraphQL endpoint
+app.UseGraphQL<BooksSchema>("/graphql");
+
+// GraphQL UI (development only) - use external tools like Altair, Insomnia, or GraphiQL
+if (app.Environment.IsDevelopment())
+{
+    app.UseGraphQLAltair("/graphql/ui");
+}
+
+app.Run();
--- a/backend/Books.Api/Properties/launchSettings.json
+++ b/backend/Books.Api/Properties/launchSettings.json
@ -0,0 +1,23 @@
+{
+  "$schema": "https://json.schemastore.org/launchsettings.json",
+  "profiles": {
+    "http": {
+      "commandName": "Project",
+      "dotnetRunMessages": true,
+      "launchBrowser": false,
+      "applicationUrl": "http://localhost:5142",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    },
+    "https": {
+      "commandName": "Project",
+      "dotnetRunMessages": true,
+      "launchBrowser": false,
+      "applicationUrl": "https://localhost:7141;http://localhost:5142",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    }
+  }
+}
--- a/backend/Books.Api/Startup.cs
+++ b/backend/Books.Api/Startup.cs
@ -0,0 +1,77 @@
+using Books.Api.EventFlow.Extensions;
+using Books.Api.EventFlow.Infrastructure;
+using Books.Api.GraphQL;
+using Books.Api.Infrastructure;
+using Books.Api.Logging;
+using EventFlow;
+using EventFlow.Configuration;
+using EventFlow.Extensions;
+using EventFlow.Hangfire.Extensions;
+using EventFlow.PostgreSql.Connections;
+using EventFlow.PostgreSql.Extensions;
+using EventFlow.ReadStores;
+using EventFlow.Subscribers;
+using GraphQL;
+using Hangfire;
+using Hangfire.PostgreSql;
+using Npgsql;
+
+namespace Books.Api;
+
+public static class Startup
+{
+    public static void ConfigureServices(IServiceCollection services, IConfiguration config, IHostEnvironment? environment = null)
+    {
+        var connectionString = config.GetConnectionString("Default")
+            ?? throw new InvalidOperationException("Connection string 'Default' not found");
+
+        // Run database migrations (skipped in Test environment where migrations run separately with test connection string)
+        var isTestEnvironment = environment?.EnvironmentName == "Test";
+        if (!isTestEnvironment)
+        {
+            DatabaseMigrator.Migrate(connectionString);
+        }
+
+        // PostgreSQL data source
+        var dataSource = new NpgsqlDataSourceBuilder(connectionString).Build();
+        services.AddSingleton(dataSource);
+
+        // Hangfire
+        services.AddHangfire(c => c
+            .SetDataCompatibilityLevel(CompatibilityLevel.Version_180)
+            .UseSimpleAssemblyNameTypeSerializer()
+            .UseRecommendedSerializerSettings()
+            .UsePostgreSqlStorage(o => o.UseNpgsqlConnection(connectionString)));
+        services.AddHangfireServer();
+
+        // Scheduler abstraction over Hangfire
+        services.AddSingleton<IScheduler, HangfireScheduler>();
+
+        // EventFlow
+        services.AddEventFlow(o => o
+            .UsePostgreSqlEventStore()
+            .ConfigurePostgreSql(PostgreSqlConfiguration.New.SetConnectionString(connectionString))
+            .AddDefaults(typeof(Startup).Assembly)
+            .AddReadModels()
+            .Configure(c => c.IsAsynchronousSubscribersEnabled = true)
+            .UseHangfireJobScheduler());
+
+        // Resilience strategies
+        services.AddSingleton<IDispatchToReadStoresResilienceStrategy, ReadStoresResilienceStrategy>();
+        services.AddSingleton<IDispatchToSubscriberResilienceStrategy, DispatchToSubscriberResilienceStrategy>();
+
+        // Read model repositories
+        services.AddRepositories();
+
+        // Logging decorators
+        services.DecorateAsyncEventHandlersWithLogging();
+
+        // GraphQL
+        services.AddGraphQL(builder => builder
+            .AddSchema<BooksSchema>()
+            .AddSystemTextJson()
+            .AddDataLoader()
+            .AddGraphTypes(typeof(BooksSchema).Assembly)
+            .AddErrorInfoProvider(opt => opt.ExposeExceptionDetails = true));
+    }
+}
--- a/backend/Books.Api/WeatherForecast.cs
+++ b/backend/Books.Api/WeatherForecast.cs
@ -0,0 +1,12 @@
+namespace Books.Api;
+
+public class WeatherForecast
+{
+    public DateOnly Date { get; set; }
+
+    public int TemperatureC { get; set; }
+
+    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);
+
+    public string? Summary { get; set; }
+}
--- a/backend/Books.Api/appsettings.json
+++ b/backend/Books.Api/appsettings.json
@ -0,0 +1,14 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning",
+      "EventFlow": "Information",
+      "Hangfire": "Information"
+    }
+  },
+  "AllowedHosts": "*",
+  "ConnectionStrings": {
+    "Default": "Host=localhost;Database=books;Username=postgres;Password=postgres;Include Error Detail=true"
+  }
+}
--- a/backend/Books.slnx
+++ b/backend/Books.slnx
@ -0,0 +1,11 @@
+<Solution>
+  <Project Path="..\..\enable-banking-access\src\EnableBanking.Client\EnableBanking.Client.csproj" Type="Classic C#" />
+  <Project Path="..\..\enable-banking-access\src\EnableBanking.Core\EnableBanking.Core.csproj" Type="Classic C#" />
+  <Project Path="..\..\enable-banking-access\src\EnableBanking.Persistence.Postgres\EnableBanking.Persistence.Postgres.csproj" Type="Classic C#" />
+  <Project Path="..\..\enable-banking-access\tests\EnableBanking.Client.Tests\EnableBanking.Client.Tests.csproj" Type="Classic C#" />
+  <Project Path="..\..\enable-banking-access\tests\EnableBanking.Core.Tests\EnableBanking.Core.Tests.csproj" Type="Classic C#" />
+  <Project Path="..\..\ledger\src\Ledger.Core\Ledger.Core.csproj" Type="Classic C#" />
+  <Project Path="..\..\ledger\src\Ledger.Infrastructure\Ledger.Infrastructure.csproj" Type="Classic C#" />
+  <Project Path="Books.Api.Tests\Books.Api.Tests.csproj" Type="Classic C#" />
+  <Project Path="Books.Api/Books.Api.csproj" />
+</Solution>
--- a/frontend/index.html
+++ b/frontend/index.html
@ -0,0 +1,13 @@
+<!doctype html>
+<html lang="da">
+  <head>
+    <meta charset="UTF-8" />
+    <link rel="icon" type="image/svg+xml" href="/vite.svg" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>Bogfoering - Regnskabssystem</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/main.tsx"></script>
+  </body>
+</html>
--- a/frontend/package-lock.json
+++ b/frontend/package-lock.json
--- a/frontend/package.json
+++ b/frontend/package.json
@ -0,0 +1,40 @@
+{
+  "name": "books",
+  "version": "1.0.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "lint": "eslint .",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "@ant-design/charts": "^2.2.1",
+    "@ant-design/icons": "^5.5.1",
+    "@tanstack/react-query": "^5.62.7",
+    "antd": "^5.22.3",
+    "dayjs": "^1.11.13",
+    "graphql": "^16.9.0",
+    "graphql-request": "^7.1.2",
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1",
+    "react-router-dom": "^7.1.1",
+    "xlsx": "^0.18.5",
+    "zustand": "^5.0.2"
+  },
+  "devDependencies": {
+    "@eslint/js": "^9.17.0",
+    "@types/node": "^22.10.2",
+    "@types/react": "^18.3.17",
+    "@types/react-dom": "^18.3.5",
+    "@vitejs/plugin-react": "^4.3.4",
+    "eslint": "^9.17.0",
+    "eslint-plugin-react-hooks": "^5.1.0",
+    "eslint-plugin-react-refresh": "^0.4.16",
+    "globals": "^15.14.0",
+    "typescript": "~5.6.2",
+    "typescript-eslint": "^8.18.2",
+    "vite": "^6.0.5"
+  }
+}
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@ -0,0 +1,18 @@
+import { BrowserRouter } from 'react-router-dom';
+import { App as AntApp } from 'antd';
+import AppRoutes from './routes';
+import AppLayout from './components/layout/AppLayout';
+
+function App() {
+  return (
+    <AntApp>
+      <BrowserRouter>
+        <AppLayout>
+          <AppRoutes />
+        </AppLayout>
+      </BrowserRouter>
+    </AntApp>
+  );
+}
+
+export default App;
--- a/frontend/src/api/client.ts
+++ b/frontend/src/api/client.ts
@ -0,0 +1,73 @@
+import { GraphQLClient } from 'graphql-request';
+import { QueryClient } from '@tanstack/react-query';
+
+// GraphQL endpoint - configure based on environment
+const GRAPHQL_ENDPOINT = import.meta.env.VITE_GRAPHQL_ENDPOINT || 'http://localhost:5000/graphql';
+
+// Create GraphQL client
+export const graphqlClient = new GraphQLClient(GRAPHQL_ENDPOINT, {
+  headers: {
+    // Add auth headers here when authentication is implemented
+    // 'Authorization': `Bearer ${token}`,
+  },
+});
+
+// Configure headers dynamically (for auth tokens, etc.)
+export const setAuthHeader = (token: string) => {
+  graphqlClient.setHeader('Authorization', `Bearer ${token}`);
+};
+
+export const removeAuthHeader = () => {
+  graphqlClient.setHeader('Authorization', '');
+};
+
+// Create TanStack Query client with default options
+export const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      // Cache data for 5 minutes
+      staleTime: 5 * 60 * 1000,
+      // Keep unused data in cache for 30 minutes
+      gcTime: 30 * 60 * 1000,
+      // Retry failed requests 3 times
+      retry: 3,
+      // Retry delay with exponential backoff
+      retryDelay: (attemptIndex) => Math.min(1000 * 2 ** attemptIndex, 30000),
+      // Refetch on window focus for fresh data
+      refetchOnWindowFocus: true,
+    },
+    mutations: {
+      // Retry mutations once
+      retry: 1,
+    },
+  },
+});
+
+// Helper function to make GraphQL requests with error handling
+export async function fetchGraphQL<TData, TVariables extends Record<string, unknown>>(
+  query: string,
+  variables?: TVariables
+): Promise<TData> {
+  try {
+    const data = await graphqlClient.request<TData>(query, variables);
+    return data;
+  } catch (error) {
+    // Log error for debugging
+    console.error('GraphQL Error:', error);
+
+    // Re-throw with more context
+    if (error instanceof Error) {
+      throw new Error(`GraphQL request failed: ${error.message}`);
+    }
+    throw error;
+  }
+}
+
+// Type-safe query helper
+export function createQueryKey(base: string, params?: Record<string, unknown>): string[] {
+  const key = [base];
+  if (params) {
+    key.push(JSON.stringify(params));
+  }
+  return key;
+}
--- a/frontend/src/components/layout/AppLayout.tsx
+++ b/frontend/src/components/layout/AppLayout.tsx
@ -0,0 +1,40 @@
+import { Layout } from 'antd';
+import { ReactNode } from 'react';
+import Sidebar from './Sidebar';
+import Header from './Header';
+import { useUIStore } from '@/stores/uiStore';
+
+const { Content } = Layout;
+
+interface AppLayoutProps {
+  children: ReactNode;
+}
+
+export default function AppLayout({ children }: AppLayoutProps) {
+  const sidebarCollapsed = useUIStore((state) => state.sidebarCollapsed);
+
+  return (
+    <Layout style={{ minHeight: '100vh' }}>
+      <Sidebar />
+      <Layout
+        style={{
+          marginLeft: sidebarCollapsed ? 80 : 220,
+          transition: 'margin-left 0.2s',
+        }}
+      >
+        <Header />
+        <Content
+          style={{
+            margin: '16px',
+            padding: '16px',
+            background: '#fff',
+            borderRadius: 8,
+            minHeight: 'calc(100vh - 64px - 32px)',
+          }}
+        >
+          {children}
+        </Content>
+      </Layout>
+    </Layout>
+  );
+}
--- a/frontend/src/components/layout/CompanySwitcher.tsx
+++ b/frontend/src/components/layout/CompanySwitcher.tsx
@ -0,0 +1,94 @@
+import { Select, Space, Typography, Tag } from 'antd';
+import { ShopOutlined } from '@ant-design/icons';
+import { useCompanyStore } from '@/stores/companyStore';
+import { formatCVR } from '@/lib/formatters';
+import type { Company } from '@/types/accounting';
+
+const { Text } = Typography;
+
+// Mock data - will be replaced with API call
+const mockCompanies: Company[] = [
+  {
+    id: '1',
+    name: 'Demo Virksomhed ApS',
+    cvr: '12345678',
+    address: 'Hovedgaden 1',
+    city: 'Koebenhavn',
+    postalCode: '1000',
+    country: 'DK',
+    fiscalYearStart: 1,
+    currency: 'DKK',
+    createdAt: '2024-01-01',
+    updatedAt: '2024-01-01',
+  },
+  {
+    id: '2',
+    name: 'Anden Virksomhed A/S',
+    cvr: '87654321',
+    address: 'Sidegaden 2',
+    city: 'Aarhus',
+    postalCode: '8000',
+    country: 'DK',
+    fiscalYearStart: 7,
+    currency: 'DKK',
+    createdAt: '2024-01-01',
+    updatedAt: '2024-01-01',
+  },
+];
+
+export default function CompanySwitcher() {
+  const { activeCompany, setActiveCompany, setCompanies } = useCompanyStore();
+
+  // Initialize with mock data if needed
+  if (useCompanyStore.getState().companies.length === 0) {
+    setCompanies(mockCompanies);
+    if (!activeCompany) {
+      setActiveCompany(mockCompanies[0]);
+    }
+  }
+
+  const companies = useCompanyStore((state) => state.companies);
+
+  const handleCompanyChange = (companyId: string) => {
+    const company = companies.find((c) => c.id === companyId);
+    if (company) {
+      setActiveCompany(company);
+    }
+  };
+
+  return (
+    <Space>
+      <ShopOutlined style={{ fontSize: 18, color: '#1677ff' }} />
+      <Select
+        value={activeCompany?.id}
+        onChange={handleCompanyChange}
+        style={{ minWidth: 280 }}
+        optionLabelProp="label"
+        popupMatchSelectWidth={false}
+        options={companies.map((company) => ({
+          value: company.id,
+          label: company.name,
+          company,
+        }))}
+        optionRender={(option) => {
+          const company = option.data.company as Company;
+          return (
+            <Space direction="vertical" size={0} style={{ padding: '4px 0' }}>
+              <Space>
+                <Text strong>{company.name}</Text>
+                {company.id === activeCompany?.id && (
+                  <Tag color="blue" style={{ marginLeft: 8 }}>
+                    Aktiv
+                  </Tag>
+                )}
+              </Space>
+              <Text type="secondary" style={{ fontSize: 12 }}>
+                CVR: {formatCVR(company.cvr)}
+              </Text>
+            </Space>
+          );
+        }}
+      />
+    </Space>
+  );
+}
--- a/frontend/src/components/layout/FiscalYearSelector.tsx
+++ b/frontend/src/components/layout/FiscalYearSelector.tsx
@ -0,0 +1,242 @@
+// FiscalYearSelector - Dropdown for selecting active fiscal year (regnskabsår)
+
+import { useState, useEffect } from 'react';
+import { Select, Space, Typography, Tag, Divider, Button } from 'antd';
+import {
+  CalendarOutlined,
+  PlusOutlined,
+  SettingOutlined,
+  CheckCircleOutlined,
+  MinusCircleOutlined,
+  LockOutlined,
+} from '@ant-design/icons';
+import { usePeriodStore } from '@/stores/periodStore';
+import type { FiscalYear } from '@/types/periods';
+import { formatDateShort } from '@/lib/formatters';
+import CreateFiscalYearModal from '@/components/modals/CreateFiscalYearModal';
+
+const { Text } = Typography;
+
+// Mock data - will be replaced with API call
+const mockFiscalYears: FiscalYear[] = [
+  {
+    id: 'fy-2025',
+    companyId: '1',
+    name: '2025',
+    startDate: '2025-01-01',
+    endDate: '2025-12-31',
+    status: 'open',
+    openingBalancePosted: true,
+    createdAt: '2025-01-01',
+    updatedAt: '2025-01-01',
+  },
+  {
+    id: 'fy-2024',
+    companyId: '1',
+    name: '2024',
+    startDate: '2024-01-01',
+    endDate: '2024-12-31',
+    status: 'closed',
+    closingDate: '2025-01-15',
+    closedBy: 'user-1',
+    openingBalancePosted: true,
+    createdAt: '2024-01-01',
+    updatedAt: '2025-01-15',
+  },
+  {
+    id: 'fy-2023',
+    companyId: '1',
+    name: '2023',
+    startDate: '2023-01-01',
+    endDate: '2023-12-31',
+    status: 'locked',
+    closingDate: '2024-01-20',
+    closedBy: 'user-1',
+    openingBalancePosted: true,
+    createdAt: '2023-01-01',
+    updatedAt: '2024-01-20',
+  },
+];
+
+/**
+ * Status badge configuration
+ */
+const STATUS_CONFIG: Record<FiscalYear['status'], {
+  color: string;
+  icon: React.ReactNode;
+  label: string;
+}> = {
+  open: {
+    color: 'success',
+    icon: <CheckCircleOutlined />,
+    label: 'Åben',
+  },
+  closed: {
+    color: 'warning',
+    icon: <MinusCircleOutlined />,
+    label: 'Lukket',
+  },
+  locked: {
+    color: 'error',
+    icon: <LockOutlined />,
+    label: 'Låst',
+  },
+};
+
+interface FiscalYearSelectorProps {
+  onCreateNew?: () => void;
+  onManage?: () => void;
+}
+
+export default function FiscalYearSelector({ onCreateNew, onManage }: FiscalYearSelectorProps) {
+  const {
+    fiscalYears,
+    currentFiscalYear,
+    setFiscalYears,
+    setCurrentFiscalYear,
+  } = usePeriodStore();
+
+  const [createModalOpen, setCreateModalOpen] = useState(false);
+
+  // Initialize with mock data if needed (will be replaced with API call)
+  useEffect(() => {
+    if (fiscalYears.length === 0) {
+      setFiscalYears(mockFiscalYears);
+    }
+  }, [fiscalYears.length, setFiscalYears]);
+
+  // Set default fiscal year if none selected
+  useEffect(() => {
+    if (fiscalYears.length > 0 && !currentFiscalYear) {
+      // Default to most recent open year, or first year
+      const openYear = fiscalYears.find(y => y.status === 'open');
+      setCurrentFiscalYear(openYear || fiscalYears[0]);
+    }
+  }, [fiscalYears, currentFiscalYear, setCurrentFiscalYear]);
+
+  const handleFiscalYearChange = (yearId: string) => {
+    const year = fiscalYears.find((y) => y.id === yearId);
+    if (year) {
+      setCurrentFiscalYear(year);
+    }
+  };
+
+  const handleCreateNew = () => {
+    if (onCreateNew) {
+      onCreateNew();
+    } else {
+      setCreateModalOpen(true);
+    }
+  };
+
+  const handleCloseCreateModal = () => {
+    setCreateModalOpen(false);
+  };
+
+  const handleCreateSuccess = (newYear: FiscalYear) => {
+    setCurrentFiscalYear(newYear);
+    setCreateModalOpen(false);
+  };
+
+  const handleManage = () => {
+    if (onManage) {
+      onManage();
+    } else {
+      // Navigate to settings page
+      console.log('Navigate to fiscal year settings');
+    }
+  };
+
+  // Sort fiscal years by start date descending (newest first)
+  const sortedYears = [...fiscalYears].sort(
+    (a, b) => new Date(b.startDate).getTime() - new Date(a.startDate).getTime()
+  );
+
+  return (
+    <Space>
+      <CalendarOutlined style={{ fontSize: 16, color: '#8c8c8c' }} />
+      <Select
+        value={currentFiscalYear?.id}
+        onChange={handleFiscalYearChange}
+        style={{ minWidth: 200 }}
+        optionLabelProp="label"
+        popupMatchSelectWidth={false}
+        dropdownRender={(menu) => (
+          <>
+            {menu}
+            <Divider style={{ margin: '8px 0' }} />
+            <Space style={{ padding: '0 8px 8px' }}>
+              <Button
+                type="text"
+                icon={<PlusOutlined />}
+                onClick={handleCreateNew}
+                size="small"
+              >
+                Opret nyt regnskabsår
+              </Button>
+              <Button
+                type="text"
+                icon={<SettingOutlined />}
+                onClick={handleManage}
+                size="small"
+              >
+                Administrer
+              </Button>
+            </Space>
+          </>
+        )}
+        options={sortedYears.map((year) => ({
+          value: year.id,
+          label: `Regnskabsår ${year.name}`,
+          year,
+        }))}
+        optionRender={(option) => {
+          const year = option.data.year;
+          // Type guard - ensure year exists and has required properties
+          if (!year || typeof year !== 'object' || !('status' in year)) {
+            return null;
+          }
+          const fiscalYear = year as FiscalYear;
+          const statusConfig = STATUS_CONFIG[fiscalYear.status];
+
+          return (
+            <Space
+              direction="vertical"
+              size={0}
+              style={{ padding: '4px 0', width: '100%' }}
+            >
+              <Space style={{ width: '100%', justifyContent: 'space-between' }}>
+                <Text strong>{fiscalYear.name}</Text>
+                <Tag
+                  color={statusConfig.color}
+                  icon={statusConfig.icon}
+                  style={{ marginLeft: 8 }}
+                >
+                  {statusConfig.label}
+                </Tag>
+              </Space>
+              <Text type="secondary" style={{ fontSize: 12 }}>
+                {formatDateShort(fiscalYear.startDate)} - {formatDateShort(fiscalYear.endDate)}
+              </Text>
+            </Space>
+          );
+        }}
+      />
+      {currentFiscalYear && (
+        <Tag
+          color={STATUS_CONFIG[currentFiscalYear.status].color}
+          icon={STATUS_CONFIG[currentFiscalYear.status].icon}
+        >
+          {STATUS_CONFIG[currentFiscalYear.status].label}
+        </Tag>
+      )}
+
+      {/* Create Fiscal Year Modal */}
+      <CreateFiscalYearModal
+        open={createModalOpen}
+        onClose={handleCloseCreateModal}
+        onSuccess={handleCreateSuccess}
+      />
+    </Space>
+  );
+}
--- a/frontend/src/components/layout/Header.tsx
+++ b/frontend/src/components/layout/Header.tsx
@ -0,0 +1,113 @@
+import { Layout, Space, Button, Dropdown, Avatar, Typography, Divider } from 'antd';
+import {
+  UserOutlined,
+  LogoutOutlined,
+  SettingOutlined,
+  BellOutlined,
+  QuestionCircleOutlined,
+} from '@ant-design/icons';
+import type { MenuProps } from 'antd';
+import CompanySwitcher from './CompanySwitcher';
+import FiscalYearSelector from './FiscalYearSelector';
+
+const { Header: AntHeader } = Layout;
+const { Text } = Typography;
+
+export default function Header() {
+  const userMenuItems: MenuProps['items'] = [
+    {
+      key: 'profile',
+      icon: <UserOutlined />,
+      label: 'Min profil',
+    },
+    {
+      key: 'settings',
+      icon: <SettingOutlined />,
+      label: 'Indstillinger',
+    },
+    {
+      type: 'divider',
+    },
+    {
+      key: 'logout',
+      icon: <LogoutOutlined />,
+      label: 'Log ud',
+      danger: true,
+    },
+  ];
+
+  const handleUserMenuClick: MenuProps['onClick'] = ({ key }) => {
+    switch (key) {
+      case 'logout':
+        // Handle logout
+        console.log('Logout clicked');
+        break;
+      case 'settings':
+        // Navigate to settings
+        break;
+      case 'profile':
+        // Navigate to profile
+        break;
+    }
+  };
+
+  return (
+    <AntHeader
+      style={{
+        padding: '0 24px',
+        background: '#fff',
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'space-between',
+        borderBottom: '1px solid #f0f0f0',
+        position: 'sticky',
+        top: 0,
+        zIndex: 10,
+      }}
+    >
+      {/* Left side - Company Switcher and Fiscal Year Selector */}
+      <Space split={<Divider type="vertical" style={{ height: 24, margin: '0 8px' }} />}>
+        <CompanySwitcher />
+        <FiscalYearSelector />
+      </Space>
+
+      {/* Right side - User actions */}
+      <Space size="middle">
+        {/* Help */}
+        <Button
+          type="text"
+          icon={<QuestionCircleOutlined />}
+          title="Hjaelp"
+        />
+
+        {/* Notifications */}
+        <Button
+          type="text"
+          icon={<BellOutlined />}
+          title="Notifikationer"
+        />
+
+        {/* User Menu */}
+        <Dropdown
+          menu={{
+            items: userMenuItems,
+            onClick: handleUserMenuClick,
+          }}
+          placement="bottomRight"
+          trigger={['click']}
+        >
+          <Space style={{ cursor: 'pointer' }}>
+            <Avatar
+              size="small"
+              icon={<UserOutlined />}
+              style={{ backgroundColor: '#1677ff' }}
+            />
+            <Text style={{ maxWidth: 120 }} ellipsis>
+              Bruger
+            </Text>
+          </Space>
+        </Dropdown>
+      </Space>
+    </AntHeader>
+  );
+}
--- a/frontend/src/components/layout/Sidebar.tsx
+++ b/frontend/src/components/layout/Sidebar.tsx
@ -0,0 +1,135 @@
+import { Layout, Menu } from 'antd';
+import {
+  DashboardOutlined,
+  BookOutlined,
+  BankOutlined,
+  AccountBookOutlined,
+  PercentageOutlined,
+  TeamOutlined,
+  SettingOutlined,
+  FileTextOutlined,
+  ThunderboltOutlined,
+} from '@ant-design/icons';
+import { useNavigate, useLocation } from 'react-router-dom';
+import { useUIStore } from '@/stores/uiStore';
+import type { MenuProps } from 'antd';
+
+const { Sider } = Layout;
+
+type MenuItem = Required<MenuProps>['items'][number];
+
+function getItem(
+  label: React.ReactNode,
+  key: string,
+  icon?: React.ReactNode,
+  children?: MenuItem[]
+): MenuItem {
+  return {
+    key,
+    icon,
+    children,
+    label,
+  } as MenuItem;
+}
+
+const menuItems: MenuItem[] = [
+  getItem('Dashboard', '/', <DashboardOutlined />),
+
+  getItem('Bogfoering', 'accounting', <BookOutlined />, [
+    getItem('Hurtig Bogfoering', '/hurtig-bogforing', <ThunderboltOutlined />),
+    getItem('Kassekladde', '/kassekladde', <FileTextOutlined />),
+    getItem('Kontooversigt', '/kontooversigt', <AccountBookOutlined />),
+  ]),
+
+  getItem('Bank', 'bank', <BankOutlined />, [
+    getItem('Bankafstemning', '/bankafstemning', <BankOutlined />),
+  ]),
+
+  getItem('Rapportering', 'reporting', <PercentageOutlined />, [
+    getItem('Momsindberetning', '/momsindberetning', <PercentageOutlined />),
+    getItem('Loenforstaelse', '/loenforstaelse', <TeamOutlined />),
+  ]),
+
+  getItem('Indstillinger', '/indstillinger', <SettingOutlined />),
+];
+
+export default function Sidebar() {
+  const navigate = useNavigate();
+  const location = useLocation();
+  const { sidebarCollapsed, toggleSidebar } = useUIStore();
+
+  const handleMenuClick: MenuProps['onClick'] = ({ key }) => {
+    if (key.startsWith('/')) {
+      navigate(key);
+    }
+  };
+
+  // Determine selected keys based on current path
+  const selectedKeys = [location.pathname];
+
+  // Determine open keys for submenus
+  const getOpenKeys = () => {
+    const path = location.pathname;
+    if (path === '/kassekladde' || path === '/kontooversigt' || path === '/hurtig-bogforing') {
+      return ['accounting'];
+    }
+    if (path === '/bankafstemning') {
+      return ['bank'];
+    }
+    if (path === '/momsindberetning' || path === '/loenforstaelse') {
+      return ['reporting'];
+    }
+    return [];
+  };
+
+  return (
+    <Sider
+      collapsible
+      collapsed={sidebarCollapsed}
+      onCollapse={toggleSidebar}
+      width={220}
+      style={{
+        overflow: 'auto',
+        height: '100vh',
+        position: 'fixed',
+        left: 0,
+        top: 0,
+        bottom: 0,
+        zIndex: 100,
+      }}
+    >
+      {/* Logo */}
+      <div
+        style={{
+          height: 64,
+          display: 'flex',
+          alignItems: 'center',
+          justifyContent: 'center',
+          borderBottom: '1px solid rgba(255, 255, 255, 0.1)',
+        }}
+      >
+        <span
+          style={{
+            color: '#fff',
+            fontSize: sidebarCollapsed ? 16 : 18,
+            fontWeight: 600,
+            whiteSpace: 'nowrap',
+          }}
+        >
+          {sidebarCollapsed ? 'B' : 'Bogfoering'}
+        </span>
+      </div>
+
+      {/* Navigation Menu */}
+      <Menu
+        theme="dark"
+        mode="inline"
+        selectedKeys={selectedKeys}
+        defaultOpenKeys={getOpenKeys()}
+        items={menuItems}
+        onClick={handleMenuClick}
+        style={{ borderRight: 0 }}
+      />
+    </Sider>
+  );
+}
--- a/frontend/src/components/modals/CloseFiscalYearWizard.tsx
+++ b/frontend/src/components/modals/CloseFiscalYearWizard.tsx
@ -0,0 +1,521 @@
+// CloseFiscalYearWizard - Multi-step wizard for year-end closing (årsafslutning)
+
+import { useState, useEffect, useMemo } from 'react';
+import {
+  Modal,
+  Steps,
+  Button,
+  Alert,
+  Typography,
+  Table,
+  Statistic,
+  Row,
+  Col,
+  Card,
+  Select,
+  Divider,
+  Checkbox,
+  Result,
+  Tag,
+  Form,
+} from 'antd';
+import {
+  CheckCircleOutlined,
+  WarningOutlined,
+  LockOutlined,
+  ArrowRightOutlined,
+  ExclamationCircleOutlined,
+} from '@ant-design/icons';
+import { usePeriodStore } from '@/stores/periodStore';
+import {
+  calculateClosingSummary,
+  validateFiscalYearClose,
+  generateClosingEntries,
+  calculateClosingBalances,
+  type FiscalYearClosingSummary,
+  type GeneratedClosingEntry,
+} from '@/lib/fiscalYear';
+import { formatCurrency } from '@/lib/formatters';
+import type { FiscalYear } from '@/types/periods';
+import type { Account, Transaction } from '@/types/accounting';
+
+const { Text, Title, Paragraph } = Typography;
+
+interface CloseFiscalYearWizardProps {
+  open: boolean;
+  fiscalYear: FiscalYear;
+  accounts: Account[];
+  transactions: Transaction[];
+  onClose: () => void;
+  onSuccess?: () => void;
+}
+
+type WizardStep = 'validation' | 'summary' | 'transfer' | 'confirm' | 'complete';
+
+// Default "Overført resultat" account (equity)
+const DEFAULT_RESULT_ACCOUNT = {
+  id: 'acc-3900',
+  accountNumber: '3900',
+  name: 'Overført resultat',
+};
+
+export default function CloseFiscalYearWizard({
+  open,
+  fiscalYear,
+  accounts,
+  transactions,
+  onClose,
+  onSuccess,
+}: CloseFiscalYearWizardProps) {
+  const [currentStep, setCurrentStep] = useState<WizardStep>('validation');
+  const [isSubmitting, setIsSubmitting] = useState(false);
+  const [resultAccountId, setResultAccountId] = useState<string>(DEFAULT_RESULT_ACCOUNT.id);
+  const [createNextYear, setCreateNextYear] = useState(true);
+  const [closeOpenPeriods, setCloseOpenPeriods] = useState(true);
+  const [confirmLock, setConfirmLock] = useState(false);
+
+  const {
+    periods,
+    closeFiscalYear,
+    lockFiscalYear,
+    closePeriod,
+    lockPeriod,
+  } = usePeriodStore();
+
+  // Reset wizard when opened
+  useEffect(() => {
+    if (open) {
+      setCurrentStep('validation');
+      setIsSubmitting(false);
+      setConfirmLock(false);
+    }
+  }, [open]);
+
+  // Calculate closing balances
+  const closingBalances = useMemo(
+    () => calculateClosingBalances(fiscalYear, accounts, transactions),
+    [fiscalYear, accounts, transactions]
+  );
+
+  // Calculate summary
+  const summary: FiscalYearClosingSummary = useMemo(
+    () => calculateClosingSummary(fiscalYear, periods, closingBalances, transactions),
+    [fiscalYear, periods, closingBalances, transactions]
+  );
+
+  // Validate fiscal year close
+  const validation = useMemo(
+    () => validateFiscalYearClose(fiscalYear, periods),
+    [fiscalYear, periods]
+  );
+
+  // Generate closing entries preview
+  const closingEntries: GeneratedClosingEntry[] = useMemo(() => {
+    const resultAccount = accounts.find((a) => a.id === resultAccountId) || {
+      id: resultAccountId,
+      accountNumber: DEFAULT_RESULT_ACCOUNT.accountNumber,
+      name: DEFAULT_RESULT_ACCOUNT.name,
+    };
+
+    return generateClosingEntries(
+      fiscalYear,
+      closingBalances,
+      resultAccount.id,
+      resultAccount.accountNumber,
+      resultAccount.name
+    );
+  }, [fiscalYear, closingBalances, resultAccountId, accounts]);
+
+  // Equity accounts for result transfer
+  const equityAccounts = useMemo(
+    () => accounts.filter((a) => a.type === 'equity'),
+    [accounts]
+  );
+
+  // Year periods
+  const yearPeriods = useMemo(
+    () => periods.filter((p) => p.fiscalYearId === fiscalYear.id),
+    [periods, fiscalYear.id]
+  );
+
+  const openPeriodsInYear = yearPeriods.filter((p) => p.status === 'open');
+
+  const handleNext = () => {
+    const steps: WizardStep[] = ['validation', 'summary', 'transfer', 'confirm', 'complete'];
+    const currentIndex = steps.indexOf(currentStep);
+    if (currentIndex < steps.length - 1) {
+      setCurrentStep(steps[currentIndex + 1]);
+    }
+  };
+
+  const handlePrevious = () => {
+    const steps: WizardStep[] = ['validation', 'summary', 'transfer', 'confirm', 'complete'];
+    const currentIndex = steps.indexOf(currentStep);
+    if (currentIndex > 0) {
+      setCurrentStep(steps[currentIndex - 1]);
+    }
+  };
+
+  const handleSubmit = async () => {
+    setIsSubmitting(true);
+
+    try {
+      // 1. Close open periods if requested
+      if (closeOpenPeriods) {
+        for (const period of openPeriodsInYear) {
+          closePeriod(period.id, 'system');
+        }
+      }
+
+      // 2. Lock all periods in the year
+      for (const period of yearPeriods) {
+        lockPeriod(period.id, 'system');
+      }
+
+      // 3. Close and lock the fiscal year
+      closeFiscalYear(fiscalYear.id, 'system');
+      lockFiscalYear(fiscalYear.id, 'system');
+
+      // 4. Move to complete step
+      setCurrentStep('complete');
+
+      onSuccess?.();
+    } catch (error) {
+      console.error('Failed to close fiscal year:', error);
+    } finally {
+      setIsSubmitting(false);
+    }
+  };
+
+  const getStepNumber = (step: WizardStep): number => {
+    const steps: WizardStep[] = ['validation', 'summary', 'transfer', 'confirm', 'complete'];
+    return steps.indexOf(step);
+  };
+
+  const renderValidationStep = () => (
+    <div>
+      <Title level={5}>Validering af regnskabsår {fiscalYear.name}</Title>
+      <Paragraph type="secondary">
+        Før årsafslutning kontrolleres regnskabsåret for eventuelle problemer.
+      </Paragraph>
+
+      {validation.errors.length > 0 && (
+        <Alert
+          type="error"
+          message="Fejl fundet"
+          description={
+            <ul style={{ margin: 0, paddingLeft: 16 }}>
+              {validation.errors.map((err, idx) => (
+                <li key={idx}>{err.messageDanish}</li>
+              ))}
+            </ul>
+          }
+          style={{ marginBottom: 16 }}
+        />
+      )}
+
+      {validation.warnings.length > 0 && (
+        <Alert
+          type="warning"
+          icon={<WarningOutlined />}
+          message="Advarsler"
+          description={
+            <ul style={{ margin: 0, paddingLeft: 16 }}>
+              {validation.warnings.map((warn, idx) => (
+                <li key={idx}>{warn.messageDanish}</li>
+              ))}
+            </ul>
+          }
+          style={{ marginBottom: 16 }}
+        />
+      )}
+
+      {summary.unreconciledCount > 0 && (
+        <Alert
+          type="info"
+          icon={<ExclamationCircleOutlined />}
+          message={`${summary.unreconciledCount} transaktioner er ikke afstemt`}
+          description="Det anbefales at afstemme alle transaktioner før årsafslutning."
+          style={{ marginBottom: 16 }}
+        />
+      )}
+
+      {openPeriodsInYear.length > 0 && (
+        <Card size="small" style={{ marginBottom: 16 }}>
+          <Checkbox
+            checked={closeOpenPeriods}
+            onChange={(e) => setCloseOpenPeriods(e.target.checked)}
+          >
+            <Text>
+              Luk automatisk {openPeriodsInYear.length} åbne periode(r) ved årsafslutning
+            </Text>
+          </Checkbox>
+        </Card>
+      )}
+
+      {validation.isValid && validation.warnings.length === 0 && (
+        <Alert
+          type="success"
+          icon={<CheckCircleOutlined />}
+          message="Regnskabsåret er klar til afslutning"
+          description="Ingen fejl eller advarsler fundet."
+        />
+      )}
+    </div>
+  );
+
+  const renderSummaryStep = () => (
+    <div>
+      <Title level={5}>Resultatoversigt for {fiscalYear.name}</Title>
+
+      <Row gutter={16} style={{ marginBottom: 24 }}>
+        <Col span={8}>
+          <Card>
+            <Statistic
+              title="Samlet indtægt"
+              value={summary.totalRevenue}
+              formatter={(val) => formatCurrency(Number(val))}
+              valueStyle={{ color: '#3f8600' }}
+            />
+          </Card>
+        </Col>
+        <Col span={8}>
+          <Card>
+            <Statistic
+              title="Samlet udgift"
+              value={summary.totalExpenses}
+              formatter={(val) => formatCurrency(Number(val))}
+              valueStyle={{ color: '#cf1322' }}
+            />
+          </Card>
+        </Col>
+        <Col span={8}>
+          <Card>
+            <Statistic
+              title="Årsresultat"
+              value={summary.netResult}
+              formatter={(val) => formatCurrency(Number(val))}
+              valueStyle={{ color: summary.netResult >= 0 ? '#3f8600' : '#cf1322' }}
+              prefix={summary.netResult >= 0 ? '+' : ''}
+            />
+          </Card>
+        </Col>
+      </Row>
+
+      <Divider />
+
+      <Text type="secondary">
+        Årsresultatet på <strong>{formatCurrency(summary.netResult)}</strong> vil blive
+        overført til egenkapitalen ved årsafslutning.
+      </Text>
+    </div>
+  );
+
+  const renderTransferStep = () => (
+    <div>
+      <Title level={5}>Resultatoverførsel</Title>
+      <Paragraph type="secondary">
+        Vælg hvilken egenkapitalkonto årsresultatet skal overføres til.
+      </Paragraph>
+
+      <Form.Item label="Egenkapitalkonto for resultatoverførsel">
+        <Select
+          value={resultAccountId}
+          onChange={setResultAccountId}
+          style={{ width: '100%' }}
+          options={[
+            ...equityAccounts.map((account) => ({
+              value: account.id,
+              label: `${account.accountNumber} - ${account.name}`,
+            })),
+            {
+              value: DEFAULT_RESULT_ACCOUNT.id,
+              label: `${DEFAULT_RESULT_ACCOUNT.accountNumber} - ${DEFAULT_RESULT_ACCOUNT.name} (Standard)`,
+            },
+          ]}
+        />
+      </Form.Item>
+
+      <Alert
+        type="info"
+        message="Resultatoverførsel"
+        description={
+          <Text>
+            Årsresultatet på <strong>{formatCurrency(summary.netResult)}</strong> vil blive
+            bogført som {summary.netResult >= 0 ? 'kredit' : 'debet'} på den valgte konto.
+          </Text>
+        }
+      />
+    </div>
+  );
+
+  const renderConfirmStep = () => (
+    <div>
+      <Title level={5}>Bekræft årsafslutning</Title>
+
+      <Alert
+        type="warning"
+        icon={<LockOutlined />}
+        message="Permanent handling"
+        description="Når regnskabsåret er låst, kan det ikke genåbnes. Alle perioder vil også blive låst."
+        style={{ marginBottom: 16 }}
+      />
+
+      <Card title="Lukkeposter der oprettes" size="small" style={{ marginBottom: 16 }}>
+        <Table
+          dataSource={closingEntries.map((entry, idx) => ({ ...entry, key: idx }))}
+          columns={[
+            {
+              title: 'Type',
+              dataIndex: 'type',
+              key: 'type',
+              render: (type: string) => (
+                <Tag>
+                  {type === 'revenue-close' ? 'Luk indtægter' : 'Luk udgifter'}
+                </Tag>
+              ),
+            },
+            {
+              title: 'Beskrivelse',
+              dataIndex: 'descriptionDanish',
+              key: 'description',
+            },
+            {
+              title: 'Beløb',
+              dataIndex: 'totalAmount',
+              key: 'amount',
+              align: 'right',
+              render: (amount: number) => formatCurrency(amount),
+            },
+          ]}
+          pagination={false}
+          size="small"
+        />
+      </Card>
+
+      <Card size="small" style={{ marginBottom: 16 }}>
+        <Checkbox
+          checked={createNextYear}
+          onChange={(e) => setCreateNextYear(e.target.checked)}
+        >
+          <Text>Opret automatisk næste regnskabsår</Text>
+        </Checkbox>
+      </Card>
+
+      <Alert
+        type="error"
+        icon={<ExclamationCircleOutlined />}
+        message="Bekræft låsning"
+        description={
+          <Checkbox
+            checked={confirmLock}
+            onChange={(e) => setConfirmLock(e.target.checked)}
+            style={{ marginTop: 8 }}
+          >
+            <Text strong>
+              Jeg bekræfter at jeg vil låse regnskabsår {fiscalYear.name} permanent.
+              Dette kan ikke fortrydes.
+            </Text>
+          </Checkbox>
+        }
+      />
+    </div>
+  );
+
+  const renderCompleteStep = () => (
+    <Result
+      status="success"
+      icon={<CheckCircleOutlined />}
+      title="Årsafslutning gennemført"
+      subTitle={`Regnskabsår ${fiscalYear.name} er nu lukket og låst.`}
+      extra={[
+        <Button type="primary" key="close" onClick={onClose}>
+          Luk
+        </Button>,
+      ]}
+    />
+  );
+
+  const renderStepContent = () => {
+    switch (currentStep) {
+      case 'validation':
+        return renderValidationStep();
+      case 'summary':
+        return renderSummaryStep();
+      case 'transfer':
+        return renderTransferStep();
+      case 'confirm':
+        return renderConfirmStep();
+      case 'complete':
+        return renderCompleteStep();
+      default:
+        return null;
+    }
+  };
+
+  const canProceed = currentStep === 'validation' ? validation.isValid : true;
+  const isLastStep = currentStep === 'confirm';
+  const isComplete = currentStep === 'complete';
+
+  return (
+    <Modal
+      title={`Årsafslutning - ${fiscalYear.name}`}
+      open={open}
+      onCancel={onClose}
+      width={700}
+      footer={
+        isComplete
+          ? null
+          : [
+              <Button key="cancel" onClick={onClose}>
+                Annuller
+              </Button>,
+              currentStep !== 'validation' && (
+                <Button key="prev" onClick={handlePrevious}>
+                  Tilbage
+                </Button>
+              ),
+              isLastStep ? (
+                <Button
+                  key="submit"
+                  type="primary"
+                  danger
+                  icon={<LockOutlined />}
+                  loading={isSubmitting}
+                  disabled={!confirmLock}
+                  onClick={handleSubmit}
+                >
+                  Gennemfør årsafslutning
+                </Button>
+              ) : (
+                <Button
+                  key="next"
+                  type="primary"
+                  onClick={handleNext}
+                  disabled={!canProceed}
+                  icon={<ArrowRightOutlined />}
+                >
+                  Næste
+                </Button>
+              ),
+            ].filter(Boolean)
+      }
+      destroyOnClose
+    >
+      <Steps
+        current={getStepNumber(currentStep)}
+        size="small"
+        style={{ marginBottom: 24 }}
+        items={[
+          { title: 'Validering' },
+          { title: 'Oversigt' },
+          { title: 'Overførsel' },
+          { title: 'Bekræft' },
+          { title: 'Færdig' },
+        ]}
+      />
+
+      {renderStepContent()}
+    </Modal>
+  );
+}
--- a/frontend/src/components/modals/CreateFiscalYearModal.tsx
+++ b/frontend/src/components/modals/CreateFiscalYearModal.tsx
@ -0,0 +1,308 @@
+// CreateFiscalYearModal - Modal for creating a new fiscal year (regnskabsår)
+
+import { useState, useEffect } from 'react';
+import {
+  Modal,
+  Form,
+  DatePicker,
+  Input,
+  Select,
+  Alert,
+  Space,
+  Typography,
+  Divider,
+} from 'antd';
+import {
+  CalendarOutlined,
+  InfoCircleOutlined,
+  WarningOutlined,
+} from '@ant-design/icons';
+import dayjs from 'dayjs';
+import type { Dayjs } from 'dayjs';
+import { usePeriodStore } from '@/stores/periodStore';
+import { useCompanyStore } from '@/stores/companyStore';
+import {
+  getFiscalYearBoundaries,
+  validateFiscalYearCreation,
+} from '@/lib/fiscalYear';
+import { generateAccountingPeriods } from '@/lib/periods';
+import type { FiscalYear, PeriodFrequency } from '@/types/periods';
+
+const { Text } = Typography;
+const { RangePicker } = DatePicker;
+
+interface CreateFiscalYearModalProps {
+  open: boolean;
+  onClose: () => void;
+  onSuccess?: (fiscalYear: FiscalYear) => void;
+  suggestedDate?: string; // Pre-populate based on trigger context
+  autoCreate?: boolean; // If true, show confirmation mode for auto-creation
+}
+
+interface FormValues {
+  dateRange: [Dayjs, Dayjs];
+  name: string;
+  periodFrequency: PeriodFrequency;
+}
+
+export default function CreateFiscalYearModal({
+  open,
+  onClose,
+  onSuccess,
+  suggestedDate,
+  autoCreate = false,
+}: CreateFiscalYearModalProps) {
+  const [form] = Form.useForm<FormValues>();
+  const [isSubmitting, setIsSubmitting] = useState(false);
+  const [validationResult, setValidationResult] = useState<ReturnType<typeof validateFiscalYearCreation> | null>(null);
+
+  const { activeCompany } = useCompanyStore();
+  const { fiscalYears, addFiscalYear, setPeriods, periods, setCurrentFiscalYear } = usePeriodStore();
+
+  // Calculate suggested fiscal year boundaries
+  useEffect(() => {
+    if (open && activeCompany) {
+      const dateToUse = suggestedDate || dayjs().format('YYYY-MM-DD');
+      const boundaries = getFiscalYearBoundaries(
+        dateToUse,
+        activeCompany.fiscalYearStart
+      );
+
+      form.setFieldsValue({
+        dateRange: [dayjs(boundaries.startDate), dayjs(boundaries.endDate)],
+        name: boundaries.name,
+        periodFrequency: 'monthly',
+      });
+
+      // Validate immediately
+      handleValidation(boundaries.startDate, boundaries.endDate);
+    }
+  }, [open, activeCompany, suggestedDate, form]);
+
+  const handleValidation = (startDate: string, endDate: string) => {
+    const result = validateFiscalYearCreation(
+      { startDate, endDate },
+      fiscalYears
+    );
+    setValidationResult(result);
+  };
+
+  const handleDateRangeChange = (dates: [Dayjs | null, Dayjs | null] | null) => {
+    if (dates && dates[0] && dates[1]) {
+      const startDate = dates[0].format('YYYY-MM-DD');
+      const endDate = dates[1].format('YYYY-MM-DD');
+
+      // Auto-update name
+      const startYear = dates[0].year();
+      const endYear = dates[1].year();
+      const name = startYear === endYear ? `${startYear}` : `${startYear}/${endYear}`;
+      form.setFieldValue('name', name);
+
+      handleValidation(startDate, endDate);
+    }
+  };
+
+  const handleSubmit = async () => {
+    try {
+      const values = await form.validateFields();
+      setIsSubmitting(true);
+
+      if (!activeCompany) {
+        throw new Error('No active company');
+      }
+
+      const startDate = values.dateRange[0].format('YYYY-MM-DD');
+      const endDate = values.dateRange[1].format('YYYY-MM-DD');
+
+      // Create fiscal year object
+      const newFiscalYear: FiscalYear = {
+        id: `fy-${values.name}-${Date.now()}`,
+        companyId: activeCompany.id,
+        name: values.name,
+        startDate,
+        endDate,
+        status: 'open',
+        openingBalancePosted: false,
+        createdAt: new Date().toISOString(),
+        updatedAt: new Date().toISOString(),
+      };
+
+      // Generate accounting periods and add required fields
+      const generatedPeriods = generateAccountingPeriods(newFiscalYear, values.periodFrequency);
+      const now = new Date().toISOString();
+      const newPeriods = generatedPeriods.map((p, idx) => ({
+        ...p,
+        id: `period-${newFiscalYear.id}-${idx + 1}`,
+        createdAt: now,
+        updatedAt: now,
+      }));
+
+      // Add to store
+      addFiscalYear(newFiscalYear);
+      setPeriods([...periods, ...newPeriods]);
+
+      // Set as current if this is the first or most recent
+      const allYears = [...fiscalYears, newFiscalYear];
+      const sortedYears = allYears.sort(
+        (a, b) => new Date(b.startDate).getTime() - new Date(a.startDate).getTime()
+      );
+      if (sortedYears[0].id === newFiscalYear.id) {
+        setCurrentFiscalYear(newFiscalYear);
+      }
+
+      onSuccess?.(newFiscalYear);
+      onClose();
+    } catch (error) {
+      console.error('Failed to create fiscal year:', error);
+    } finally {
+      setIsSubmitting(false);
+    }
+  };
+
+  const hasErrors = validationResult?.errors && validationResult.errors.length > 0;
+  const hasWarnings = validationResult?.warnings && validationResult.warnings.length > 0;
+
+  return (
+    <Modal
+      title={
+        <Space>
+          <CalendarOutlined />
+          {autoCreate ? 'Opret nyt regnskabsår?' : 'Opret nyt regnskabsår'}
+        </Space>
+      }
+      open={open}
+      onCancel={onClose}
+      onOk={handleSubmit}
+      okText={autoCreate ? 'Ja, opret regnskabsår' : 'Opret regnskabsår'}
+      cancelText="Annuller"
+      okButtonProps={{
+        disabled: hasErrors || isSubmitting || !activeCompany,
+        loading: isSubmitting,
+      }}
+      width={520}
+      destroyOnClose
+    >
+      {!activeCompany && (
+        <Alert
+          type="error"
+          message="Ingen virksomhed valgt"
+          description="Du skal vælge en virksomhed før du kan oprette et regnskabsår."
+          style={{ marginBottom: 16 }}
+        />
+      )}
+
+      {autoCreate && suggestedDate && (
+        <Alert
+          type="info"
+          icon={<InfoCircleOutlined />}
+          message="Dato udenfor eksisterende regnskabsår"
+          description={
+            <Text>
+              Datoen <strong>{dayjs(suggestedDate).format('D. MMMM YYYY')}</strong> falder
+              udenfor eksisterende regnskabsår. Vil du oprette et nyt regnskabsår?
+            </Text>
+          }
+          style={{ marginBottom: 16 }}
+        />
+      )}
+
+      <Form
+        form={form}
+        layout="vertical"
+        initialValues={{
+          periodFrequency: 'monthly',
+        }}
+      >
+        <Form.Item
+          name="dateRange"
+          label="Periode"
+          rules={[{ required: true, message: 'Vælg start- og slutdato' }]}
+        >
+          <RangePicker
+            style={{ width: '100%' }}
+            format="DD-MM-YYYY"
+            onChange={handleDateRangeChange}
+          />
+        </Form.Item>
+
+        <Form.Item
+          name="name"
+          label="Navn"
+          rules={[{ required: true, message: 'Indtast navn på regnskabsåret' }]}
+        >
+          <Input
+            placeholder="f.eks. 2025 eller 2024/2025"
+            prefix={<CalendarOutlined />}
+          />
+        </Form.Item>
+
+        <Form.Item
+          name="periodFrequency"
+          label="Regnskabsperioder"
+          tooltip="Hvor ofte skal regnskabsperioder oprettes?"
+        >
+          <Select
+            options={[
+              { value: 'monthly', label: 'Månedlig (12 perioder)' },
+              { value: 'quarterly', label: 'Kvartalsvis (4 perioder)' },
+              { value: 'half-yearly', label: 'Halvårlig (2 perioder)' },
+              { value: 'yearly', label: 'Årlig (1 periode)' },
+            ]}
+          />
+        </Form.Item>
+
+        {/* Validation feedback */}
+        {hasErrors && (
+          <Alert
+            type="error"
+            message="Kan ikke oprette regnskabsår"
+            description={
+              <ul style={{ margin: 0, paddingLeft: 16 }}>
+                {validationResult?.errors.map((err, idx) => (
+                  <li key={idx}>{err.messageDanish}</li>
+                ))}
+              </ul>
+            }
+            style={{ marginTop: 16 }}
+          />
+        )}
+
+        {hasWarnings && !hasErrors && (
+          <Alert
+            type="warning"
+            icon={<WarningOutlined />}
+            message="Advarsler"
+            description={
+              <ul style={{ margin: 0, paddingLeft: 16 }}>
+                {validationResult?.warnings.map((warn, idx) => (
+                  <li key={idx}>{warn.messageDanish}</li>
+                ))}
+              </ul>
+            }
+            style={{ marginTop: 16 }}
+          />
+        )}
+
+        {validationResult?.isValid && !hasWarnings && (
+          <Alert
+            type="success"
+            message="Klar til oprettelse"
+            description="Regnskabsåret kan oprettes uden problemer."
+            style={{ marginTop: 16 }}
+          />
+        )}
+      </Form>
+
+      {activeCompany && activeCompany.fiscalYearStart !== 1 && (
+        <>
+          <Divider style={{ margin: '16px 0' }} />
+          <Text type="secondary" style={{ fontSize: 12 }}>
+            <InfoCircleOutlined style={{ marginRight: 4 }} />
+            Din virksomhed har skævt regnskabsår (starter i måned {activeCompany.fiscalYearStart}).
+            Datoerne er automatisk justeret baseret på dette.
+          </Text>
+        </>
+      )}
+    </Modal>
+  );
+}
--- a/frontend/src/components/simple-booking/AccountQuickPicker.tsx
+++ b/frontend/src/components/simple-booking/AccountQuickPicker.tsx
@ -0,0 +1,220 @@
+// AccountQuickPicker - Quick account selection with favorites
+
+import { useState, useMemo } from 'react';
+import { Select, Button, Space, Tag, Typography, Divider, Input } from 'antd';
+import { StarOutlined, StarFilled, SearchOutlined } from '@ant-design/icons';
+import type { Account } from '@/types/accounting';
+import type { VATCode } from '@/types/vat';
+import { VAT_CODE_CONFIG } from '@/lib/vatCodes';
+import { useTopFavorites, useSimpleBookingStore } from '@/stores/simpleBookingStore';
+
+const { Text } = Typography;
+
+interface AccountOption {
+  id: string;
+  accountNumber: string;
+  name: string;
+  type: string;
+}
+
+interface AccountQuickPickerProps {
+  accounts: AccountOption[];
+  value?: string;
+  onChange: (accountId: string, account: AccountOption) => void;
+  isExpense?: boolean;
+  favoriteLimit?: number;
+  placeholder?: string;
+}
+
+export function AccountQuickPicker({
+  accounts,
+  value,
+  onChange,
+  isExpense = true,
+  favoriteLimit = 6,
+  placeholder = 'Soeg efter konto...',
+}: AccountQuickPickerProps) {
+  const [searchValue, setSearchValue] = useState('');
+  const topFavorites = useTopFavorites(favoriteLimit);
+  const { addFavoriteAccount, removeFavoriteAccount, incrementFavoriteUsage, favoriteAccounts } = useSimpleBookingStore();
+
+  // Filter accounts based on search
+  const filteredAccounts = useMemo(() => {
+    if (!searchValue) return accounts;
+    const search = searchValue.toLowerCase();
+    return accounts.filter(
+      (account) =>
+        account.accountNumber.toLowerCase().includes(search) ||
+        account.name.toLowerCase().includes(search)
+    );
+  }, [accounts, searchValue]);
+
+  // Check if an account is a favorite
+  const isFavorite = (accountId: string) =>
+    favoriteAccounts.some((f) => f.accountId === accountId);
+
+  // Handle account selection
+  const handleSelect = (accountId: string) => {
+    const account = accounts.find((a) => a.id === accountId);
+    if (account) {
+      onChange(accountId, account);
+      // Increment favorite usage if it's a favorite
+      if (isFavorite(accountId)) {
+        incrementFavoriteUsage(accountId);
+      }
+    }
+  };
+
+  // Handle favorite toggle
+  const handleToggleFavorite = (account: AccountOption, event: React.MouseEvent) => {
+    event.stopPropagation();
+    if (isFavorite(account.id)) {
+      removeFavoriteAccount(account.id);
+    } else {
+      addFavoriteAccount({
+        accountId: account.id,
+        accountNumber: account.accountNumber,
+        accountName: account.name,
+        defaultVATCode: isExpense ? 'K25' : 'S25',
+      });
+    }
+  };
+
+  // Get quick buttons for favorites
+  const quickButtons = topFavorites.map((fav) => {
+    const account = accounts.find((a) => a.id === fav.accountId);
+    if (!account) return null;
+    return (
+      <Button
+        key={fav.id}
+        size="small"
+        type={value === fav.accountId ? 'primary' : 'default'}
+        onClick={() => handleSelect(fav.accountId)}
+        style={{ marginRight: 4, marginBottom: 4 }}
+      >
+        {account.accountNumber} {account.name.substring(0, 12)}
+        {account.name.length > 12 ? '...' : ''}
+      </Button>
+    );
+  }).filter(Boolean);
+
+  // Select options
+  const selectOptions = filteredAccounts.map((account) => ({
+    value: account.id,
+    label: (
+      <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center' }}>
+        <span>
+          <Text strong style={{ marginRight: 8 }}>{account.accountNumber}</Text>
+          {account.name}
+        </span>
+        <span onClick={(e) => handleToggleFavorite(account, e)} style={{ cursor: 'pointer' }}>
+          {isFavorite(account.id) ? (
+            <StarFilled style={{ color: '#faad14' }} />
+          ) : (
+            <StarOutlined style={{ color: '#d9d9d9' }} />
+          )}
+        </span>
+      </div>
+    ),
+    searchValue: `${account.accountNumber} ${account.name}`,
+  }));
+
+  return (
+    <div className="account-quick-picker">
+      {/* Quick favorite buttons */}
+      {quickButtons.length > 0 && (
+        <div style={{ marginBottom: 8 }}>
+          <Text type="secondary" style={{ fontSize: 12, display: 'block', marginBottom: 4 }}>
+            Hurtige valg:
+          </Text>
+          <div style={{ display: 'flex', flexWrap: 'wrap' }}>
+            {quickButtons}
+          </div>
+        </div>
+      )}
+
+      {/* Full search select */}
+      <Select
+        showSearch
+        style={{ width: '100%' }}
+        placeholder={placeholder}
+        value={value}
+        onChange={handleSelect}
+        onSearch={setSearchValue}
+        filterOption={false}
+        options={selectOptions}
+        optionFilterProp="searchValue"
+        suffixIcon={<SearchOutlined />}
+        notFoundContent={
+          <div style={{ padding: '8px', textAlign: 'center' }}>
+            <Text type="secondary">Ingen konti fundet</Text>
+          </div>
+        }
+      />
+    </div>
+  );
+}
+
+interface VATCodePickerProps {
+  value?: VATCode;
+  onChange: (code: VATCode) => void;
+  isExpense?: boolean;
+  showDescription?: boolean;
+}
+
+export function VATCodePicker({
+  value,
+  onChange,
+  isExpense = true,
+  showDescription = false,
+}: VATCodePickerProps) {
+  // Get relevant VAT codes based on transaction type
+  const relevantCodes = useMemo(() => {
+    if (isExpense) {
+      // Input VAT codes for expenses
+      return ['K25', 'EU_VARE', 'EU_YDELSE', 'NONE'] as VATCode[];
+    } else {
+      // Output VAT codes for income
+      return ['S25', 'MOMSFRI', 'EKSPORT', 'NONE'] as VATCode[];
+    }
+  }, [isExpense]);
+
+  const options = relevantCodes.map((code) => {
+    const config = VAT_CODE_CONFIG[code];
+    return {
+      value: code,
+      label: (
+        <div>
+          <Text strong style={{ marginRight: 8 }}>{code}</Text>
+          {config.nameDanish}
+          {config.rate > 0 && (
+            <Tag size="small" style={{ marginLeft: 8 }}>
+              {(config.rate * 100).toFixed(0)}%
+            </Tag>
+          )}
+        </div>
+      ),
+    };
+  });
+
+  const selectedConfig = value ? VAT_CODE_CONFIG[value] : null;
+
+  return (
+    <div className="vat-code-picker">
+      <Select
+        style={{ width: '100%' }}
+        placeholder="Vaelg momskode"
+        value={value}
+        onChange={onChange}
+        options={options}
+      />
+      {showDescription && selectedConfig && (
+        <Text type="secondary" style={{ fontSize: 12, display: 'block', marginTop: 4 }}>
+          {selectedConfig.description}
+        </Text>
+      )}
+    </div>
+  );
+}
+
+export default AccountQuickPicker;
--- a/frontend/src/components/simple-booking/BankTransactionCard.tsx
+++ b/frontend/src/components/simple-booking/BankTransactionCard.tsx
@ -0,0 +1,165 @@
+// BankTransactionCard - Card component for displaying a bank transaction
+
+import { Card, Button, Space, Tag, Typography, Tooltip } from 'antd';
+import { CheckOutlined, SplitCellsOutlined, BankOutlined } from '@ant-design/icons';
+import { formatCurrency, formatDateShort } from '@/lib/formatters';
+import { accountingColors } from '@/styles/theme';
+import type { PendingBankTransaction } from '@/stores/simpleBookingStore';
+
+const { Text } = Typography;
+
+interface BankTransactionCardProps {
+  transaction: PendingBankTransaction;
+  onBook: (transaction: PendingBankTransaction) => void;
+  onSplit: (transaction: PendingBankTransaction) => void;
+  isSelected?: boolean;
+  disabled?: boolean;
+}
+
+export function BankTransactionCard({
+  transaction,
+  onBook,
+  onSplit,
+  isSelected = false,
+  disabled = false,
+}: BankTransactionCardProps) {
+  const isExpense = transaction.amount < 0;
+  const amountColor = isExpense ? accountingColors.debit : accountingColors.credit;
+
+  return (
+    <Card
+      size="small"
+      className={`bank-transaction-card ${isSelected ? 'selected' : ''} ${transaction.isBooked ? 'booked' : ''}`}
+      style={{
+        marginBottom: 8,
+        borderLeft: `3px solid ${isExpense ? accountingColors.debit : accountingColors.credit}`,
+        opacity: transaction.isBooked ? 0.6 : 1,
+        background: isSelected ? '#f0f5ff' : transaction.isBooked ? '#fafafa' : '#fff',
+      }}
+    >
+      <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'flex-start' }}>
+        {/* Left: Amount and description */}
+        <div style={{ flex: 1 }}>
+          <div style={{ marginBottom: 4 }}>
+            <Text
+              strong
+              style={{
+                fontSize: 16,
+                color: amountColor,
+              }}
+            >
+              {formatCurrency(transaction.amount)}
+            </Text>
+          </div>
+          <Text
+            style={{
+              display: 'block',
+              fontSize: 13,
+              color: '#262626',
+              lineHeight: 1.4,
+            }}
+          >
+            {transaction.description}
+          </Text>
+          <div style={{ marginTop: 6 }}>
+            <Space size={4}>
+              <Text type="secondary" style={{ fontSize: 12 }}>
+                {formatDateShort(transaction.date)}
+              </Text>
+              {transaction.counterparty && (
+                <>
+                  <Text type="secondary" style={{ fontSize: 12 }}>•</Text>
+                  <Text type="secondary" style={{ fontSize: 12 }}>
+                    {transaction.counterparty}
+                  </Text>
+                </>
+              )}
+            </Space>
+          </div>
+        </div>
+
+        {/* Right: Actions */}
+        <div style={{ marginLeft: 16 }}>
+          {transaction.isBooked ? (
+            <Tag color="success" icon={<CheckOutlined />}>
+              Bogfoert
+            </Tag>
+          ) : (
+            <Space>
+              <Tooltip title="Bogfoer">
+                <Button
+                  type="primary"
+                  size="small"
+                  onClick={() => onBook(transaction)}
+                  disabled={disabled}
+                >
+                  Bogfoer
+                </Button>
+              </Tooltip>
+              <Tooltip title="Opdel paa flere konti">
+                <Button
+                  size="small"
+                  icon={<SplitCellsOutlined />}
+                  onClick={() => onSplit(transaction)}
+                  disabled={disabled}
+                >
+                  Opdel...
+                </Button>
+              </Tooltip>
+            </Space>
+          )}
+        </div>
+      </div>
+    </Card>
+  );
+}
+
+interface BankTransactionListProps {
+  transactions: PendingBankTransaction[];
+  onBook: (transaction: PendingBankTransaction) => void;
+  onSplit: (transaction: PendingBankTransaction) => void;
+  showBooked?: boolean;
+  selectedId?: string;
+  disabled?: boolean;
+}
+
+export function BankTransactionList({
+  transactions,
+  onBook,
+  onSplit,
+  showBooked = false,
+  selectedId,
+  disabled = false,
+}: BankTransactionListProps) {
+  const filteredTransactions = showBooked
+    ? transactions
+    : transactions.filter((tx) => !tx.isBooked);
+
+  if (filteredTransactions.length === 0) {
+    return (
+      <Card style={{ textAlign: 'center', padding: '24px 0' }}>
+        <BankOutlined style={{ fontSize: 32, color: '#bfbfbf', marginBottom: 8 }} />
+        <div>
+          <Text type="secondary">Ingen uboerte banktransaktioner</Text>
+        </div>
+      </Card>
+    );
+  }
+
+  return (
+    <div className="bank-transaction-list">
+      {filteredTransactions.map((transaction) => (
+        <BankTransactionCard
+          key={transaction.id}
+          transaction={transaction}
+          onBook={onBook}
+          onSplit={onSplit}
+          isSelected={transaction.id === selectedId}
+          disabled={disabled}
+        />
+      ))}
+    </div>
+  );
+}
+
+export default BankTransactionCard;
--- a/frontend/src/components/simple-booking/QuickBookModal.tsx
+++ b/frontend/src/components/simple-booking/QuickBookModal.tsx
@ -0,0 +1,298 @@
+// QuickBookModal - Modal for simple one-account booking
+
+import { useState, useEffect, useMemo, useRef } from 'react';
+import {
+  Modal,
+  Form,
+  Input,
+  Card,
+  Table,
+  Typography,
+  Alert,
+  Divider,
+  Tag,
+} from 'antd';
+import { CheckCircleOutlined, WarningOutlined } from '@ant-design/icons';
+import { formatCurrency, formatDateShort } from '@/lib/formatters';
+import { accountingColors } from '@/styles/theme';
+import { generateSimpleDoubleEntry, type SimpleBookingInput } from '@/lib/accounting';
+import type { VATCode } from '@/types/vat';
+import type { Account } from '@/types/accounting';
+import { AccountQuickPicker, VATCodePicker } from './AccountQuickPicker';
+import {
+  useBookingPreview,
+  useIsBookingSaving,
+  useSimpleBookingStore,
+} from '@/stores/simpleBookingStore';
+
+const { Text } = Typography;
+
+interface QuickBookModalProps {
+  accounts: Account[];
+  onSubmit: (transaction: ReturnType<typeof generateSimpleDoubleEntry>) => Promise<void>;
+}
+
+export function QuickBookModal({ accounts, onSubmit }: QuickBookModalProps) {
+  const { modal, closeModal, setPreview } = useSimpleBookingStore();
+  const preview = useBookingPreview();
+  const isSaving = useIsBookingSaving();
+
+  const [selectedAccountId, setSelectedAccountId] = useState<string | undefined>();
+  const [selectedVATCode, setSelectedVATCode] = useState<VATCode>('K25');
+  const [description, setDescription] = useState('');
+
+  const bankTransaction = modal.bankTransaction;
+  const isOpen = modal.isOpen && modal.type === 'simple';
+  const isExpense = bankTransaction ? bankTransaction.amount < 0 : true;
+
+  // Track previous open state to detect modal opening
+  const wasOpenRef = useRef(false);
+  const lastTransactionIdRef = useRef<string | null>(null);
+
+  // Reset form only when modal opens or transaction changes
+  useEffect(() => {
+    const isNewlyOpened = isOpen && !wasOpenRef.current;
+    const isNewTransaction = bankTransaction && bankTransaction.id !== lastTransactionIdRef.current;
+
+    if (isOpen && bankTransaction && (isNewlyOpened || isNewTransaction)) {
+      // Calculate isExpense inside effect to avoid dependency
+      const expense = bankTransaction.amount < 0;
+      setSelectedAccountId(undefined);
+      setSelectedVATCode(expense ? 'K25' : 'S25');
+      setDescription(bankTransaction.description);
+      setPreview(null);
+      lastTransactionIdRef.current = bankTransaction.id;
+    }
+
+    wasOpenRef.current = isOpen;
+  }, [isOpen, bankTransaction, setPreview]);
+
+  // Generate preview when inputs change
+  useEffect(() => {
+    if (!bankTransaction || !selectedAccountId) {
+      setPreview(null);
+      return;
+    }
+
+    const account = accounts.find((a) => a.id === selectedAccountId);
+    if (!account) {
+      setPreview(null);
+      return;
+    }
+
+    const input: SimpleBookingInput = {
+      bankTransaction: {
+        id: bankTransaction.id,
+        date: bankTransaction.date,
+        amount: bankTransaction.amount,
+        description: description || bankTransaction.description,
+        counterparty: bankTransaction.counterparty,
+        bankAccountId: bankTransaction.bankAccountId,
+        bankAccountNumber: bankTransaction.bankAccountNumber,
+      },
+      contraAccountId: account.id,
+      contraAccountNumber: account.accountNumber,
+      contraAccountName: account.name,
+      vatCode: selectedVATCode,
+      description: description || undefined,
+    };
+
+    const result = generateSimpleDoubleEntry(input);
+    setPreview(result);
+  }, [bankTransaction, selectedAccountId, selectedVATCode, description, accounts, setPreview]);
+
+  // Account options for picker
+  const accountOptions = useMemo(
+    () =>
+      accounts.map((a) => ({
+        id: a.id,
+        accountNumber: a.accountNumber,
+        name: a.name,
+        type: a.type,
+      })),
+    [accounts]
+  );
+
+  const handleAccountChange = (accountId: string) => {
+    setSelectedAccountId(accountId);
+  };
+
+  const handleSubmit = async () => {
+    if (!preview || !preview.isValid) return;
+    await onSubmit(preview);
+    closeModal();
+  };
+
+  const previewColumns = [
+    {
+      title: 'Konto',
+      key: 'account',
+      render: (_: unknown, record: { accountNumber: string; accountName: string }) => (
+        <span>
+          <Text strong>{record.accountNumber}</Text>
+          <Text style={{ marginLeft: 8 }}>{record.accountName}</Text>
+        </span>
+      ),
+    },
+    {
+      title: 'Debet',
+      dataIndex: 'debit',
+      key: 'debit',
+      align: 'right' as const,
+      render: (value: number) =>
+        value > 0 ? (
+          <Text style={{ color: accountingColors.debit }}>{formatCurrency(value)}</Text>
+        ) : null,
+    },
+    {
+      title: 'Kredit',
+      dataIndex: 'credit',
+      key: 'credit',
+      align: 'right' as const,
+      render: (value: number) =>
+        value > 0 ? (
+          <Text style={{ color: accountingColors.credit }}>{formatCurrency(value)}</Text>
+        ) : null,
+    },
+  ];
+
+  if (!bankTransaction) return null;
+
+  return (
+    <Modal
+      title="Bogfoer transaktion"
+      open={isOpen}
+      onCancel={closeModal}
+      onOk={handleSubmit}
+      okText="Bogfoer"
+      cancelText="Annuller"
+      okButtonProps={{
+        disabled: !preview?.isValid || isSaving,
+        loading: isSaving,
+      }}
+      width={600}
+      destroyOnClose
+    >
+      {/* Bank transaction summary */}
+      <Card
+        size="small"
+        style={{
+          marginBottom: 16,
+          borderLeft: `3px solid ${isExpense ? accountingColors.debit : accountingColors.credit}`,
+        }}
+      >
+        <div>
+          <Text
+            strong
+            style={{
+              fontSize: 18,
+              color: isExpense ? accountingColors.debit : accountingColors.credit,
+            }}
+          >
+            {formatCurrency(bankTransaction.amount)}
+          </Text>
+        </div>
+        <Text>{bankTransaction.description}</Text>
+        <div style={{ marginTop: 4 }}>
+          <Text type="secondary">{formatDateShort(bankTransaction.date)}</Text>
+          {bankTransaction.counterparty && (
+            <>
+              <Text type="secondary"> • </Text>
+              <Text type="secondary">{bankTransaction.counterparty}</Text>
+            </>
+          )}
+        </div>
+      </Card>
+
+      {/* Account selection */}
+      <Form layout="vertical">
+        <Form.Item label="Vaelg konto" required>
+          <AccountQuickPicker
+            accounts={accountOptions}
+            value={selectedAccountId}
+            onChange={handleAccountChange}
+            isExpense={isExpense}
+            placeholder="Soeg efter konto..."
+          />
+        </Form.Item>
+
+        <Form.Item label="Momskode" required>
+          <VATCodePicker
+            value={selectedVATCode}
+            onChange={setSelectedVATCode}
+            isExpense={isExpense}
+            showDescription
+          />
+        </Form.Item>
+
+        <Form.Item label="Beskrivelse (valgfri)">
+          <Input
+            value={description}
+            onChange={(e) => setDescription(e.target.value)}
+            placeholder={bankTransaction.description}
+          />
+        </Form.Item>
+      </Form>
+
+      {/* Preview */}
+      {preview && (
+        <>
+          <Divider style={{ margin: '16px 0' }} />
+          <div>
+            <div style={{ display: 'flex', alignItems: 'center', marginBottom: 8 }}>
+              <Text strong style={{ marginRight: 8 }}>Forhaandsvisning:</Text>
+              {preview.isValid ? (
+                <Tag color="success" icon={<CheckCircleOutlined />}>
+                  Balancerer
+                </Tag>
+              ) : (
+                <Tag color="error" icon={<WarningOutlined />}>
+                  Fejl
+                </Tag>
+              )}
+            </div>
+
+            {!preview.isValid && preview.validationMessage && (
+              <Alert
+                type="error"
+                message={preview.validationMessage}
+                style={{ marginBottom: 8 }}
+              />
+            )}
+
+            <Table
+              dataSource={preview.lines.map((line, idx) => ({ ...line, key: idx }))}
+              columns={previewColumns}
+              pagination={false}
+              size="small"
+              bordered
+              summary={() => {
+                const totalDebit = preview.lines.reduce((sum, l) => sum + l.debit, 0);
+                const totalCredit = preview.lines.reduce((sum, l) => sum + l.credit, 0);
+                return (
+                  <Table.Summary.Row>
+                    <Table.Summary.Cell index={0}>
+                      <Text strong>Total</Text>
+                    </Table.Summary.Cell>
+                    <Table.Summary.Cell index={1} align="right">
+                      <Text strong style={{ color: accountingColors.debit }}>
+                        {formatCurrency(totalDebit)}
+                      </Text>
+                    </Table.Summary.Cell>
+                    <Table.Summary.Cell index={2} align="right">
+                      <Text strong style={{ color: accountingColors.credit }}>
+                        {formatCurrency(totalCredit)}
+                      </Text>
+                    </Table.Summary.Cell>
+                  </Table.Summary.Row>
+                );
+              }}
+            />
+          </div>
+        </>
+      )}
+    </Modal>
+  );
+}
+
+export default QuickBookModal;
--- a/frontend/src/components/simple-booking/SplitBookModal.tsx
+++ b/frontend/src/components/simple-booking/SplitBookModal.tsx
@ -0,0 +1,472 @@
+// SplitBookModal - Modal for splitting one bank transaction to multiple accounts
+
+import { useState, useEffect, useMemo, useRef } from 'react';
+import {
+  Modal,
+  Form,
+  Input,
+  InputNumber,
+  Card,
+  Table,
+  Space,
+  Typography,
+  Alert,
+  Divider,
+  Tag,
+  Button,
+  Row,
+  Col,
+} from 'antd';
+import {
+  CheckCircleOutlined,
+  WarningOutlined,
+  PlusOutlined,
+  DeleteOutlined,
+} from '@ant-design/icons';
+import { formatCurrency, formatDateShort } from '@/lib/formatters';
+import { accountingColors } from '@/styles/theme';
+import { generateSplitDoubleEntry, type SplitBookingInput, type SplitBookingLine } from '@/lib/accounting';
+import type { VATCode } from '@/types/vat';
+import type { Account } from '@/types/accounting';
+import { AccountQuickPicker, VATCodePicker } from './AccountQuickPicker';
+import {
+  useBookingPreview,
+  useIsBookingSaving,
+  useSimpleBookingStore,
+} from '@/stores/simpleBookingStore';
+
+const { Text } = Typography;
+
+interface SplitBookModalProps {
+  accounts: Account[];
+  onSubmit: (transaction: ReturnType<typeof generateSplitDoubleEntry>) => Promise<void>;
+}
+
+export function SplitBookModal({ accounts, onSubmit }: SplitBookModalProps) {
+  const {
+    modal,
+    closeModal,
+    setPreview,
+    splitState,
+    addSplitLine,
+    removeSplitLine,
+    clearSplitLines,
+  } = useSimpleBookingStore();
+  const preview = useBookingPreview();
+  const isSaving = useIsBookingSaving();
+
+  // New line form state
+  const [newLineAccountId, setNewLineAccountId] = useState<string | undefined>();
+  const [newLineAmount, setNewLineAmount] = useState<number | null>(null);
+  const [newLineVATCode, setNewLineVATCode] = useState<VATCode>('K25');
+  const [newLineDescription, setNewLineDescription] = useState('');
+
+  const bankTransaction = modal.bankTransaction;
+  const isOpen = modal.isOpen && modal.type === 'split';
+  const isExpense = bankTransaction ? bankTransaction.amount < 0 : true;
+
+  // Track previous open state to detect modal opening
+  const wasOpenRef = useRef(false);
+  const lastTransactionIdRef = useRef<string | null>(null);
+
+  // Reset form only when modal opens or transaction changes
+  useEffect(() => {
+    const isNewlyOpened = isOpen && !wasOpenRef.current;
+    const isNewTransaction = bankTransaction && bankTransaction.id !== lastTransactionIdRef.current;
+
+    if (isOpen && bankTransaction && (isNewlyOpened || isNewTransaction)) {
+      // Calculate isExpense inside effect to avoid dependency
+      const expense = bankTransaction.amount < 0;
+      clearSplitLines();
+      setNewLineAccountId(undefined);
+      setNewLineAmount(null);
+      setNewLineVATCode(expense ? 'K25' : 'S25');
+      setNewLineDescription('');
+      setPreview(null);
+      lastTransactionIdRef.current = bankTransaction.id;
+    }
+
+    wasOpenRef.current = isOpen;
+  }, [isOpen, bankTransaction, clearSplitLines, setPreview]);
+
+  // Generate preview when lines change
+  useEffect(() => {
+    if (!bankTransaction || splitState.lines.length === 0) {
+      setPreview(null);
+      return;
+    }
+
+    const input: SplitBookingInput = {
+      bankTransaction: {
+        id: bankTransaction.id,
+        date: bankTransaction.date,
+        amount: bankTransaction.amount,
+        description: bankTransaction.description,
+        counterparty: bankTransaction.counterparty,
+        bankAccountId: bankTransaction.bankAccountId,
+        bankAccountNumber: bankTransaction.bankAccountNumber,
+      },
+      lines: splitState.lines,
+    };
+
+    const result = generateSplitDoubleEntry(input);
+    setPreview(result);
+  }, [bankTransaction, splitState.lines, setPreview]);
+
+  // Account options for picker
+  const accountOptions = useMemo(
+    () =>
+      accounts.map((a) => ({
+        id: a.id,
+        accountNumber: a.accountNumber,
+        name: a.name,
+        type: a.type,
+      })),
+    [accounts]
+  );
+
+  // Add a new split line
+  const handleAddLine = () => {
+    if (!newLineAccountId || !newLineAmount) return;
+
+    const account = accounts.find((a) => a.id === newLineAccountId);
+    if (!account) return;
+
+    const newLine: SplitBookingLine = {
+      accountId: account.id,
+      accountNumber: account.accountNumber,
+      accountName: account.name,
+      amount: newLineAmount,
+      vatCode: newLineVATCode,
+      description: newLineDescription || undefined,
+    };
+
+    addSplitLine(newLine);
+
+    // Reset form for next line
+    setNewLineAccountId(undefined);
+    setNewLineAmount(null);
+    setNewLineDescription('');
+  };
+
+  // Fill remaining amount
+  const handleFillRemaining = () => {
+    setNewLineAmount(splitState.remainingAmount);
+  };
+
+  const handleSubmit = async () => {
+    if (!preview || !preview.isValid) return;
+    await onSubmit(preview);
+    closeModal();
+  };
+
+  // Preview table columns
+  const previewColumns = [
+    {
+      title: 'Konto',
+      key: 'account',
+      render: (_: unknown, record: { accountNumber: string; accountName: string }) => (
+        <span>
+          <Text strong>{record.accountNumber}</Text>
+          <Text style={{ marginLeft: 8 }}>{record.accountName}</Text>
+        </span>
+      ),
+    },
+    {
+      title: 'Debet',
+      dataIndex: 'debit',
+      key: 'debit',
+      align: 'right' as const,
+      render: (value: number) =>
+        value > 0 ? (
+          <Text style={{ color: accountingColors.debit }}>{formatCurrency(value)}</Text>
+        ) : null,
+    },
+    {
+      title: 'Kredit',
+      dataIndex: 'credit',
+      key: 'credit',
+      align: 'right' as const,
+      render: (value: number) =>
+        value > 0 ? (
+          <Text style={{ color: accountingColors.credit }}>{formatCurrency(value)}</Text>
+        ) : null,
+    },
+  ];
+
+  // Split lines table columns
+  const splitLinesColumns = [
+    {
+      title: '#',
+      key: 'index',
+      width: 40,
+      render: (_: unknown, __: unknown, index: number) => index + 1,
+    },
+    {
+      title: 'Konto',
+      key: 'account',
+      render: (_: unknown, record: SplitBookingLine) => (
+        <span>
+          <Text strong>{record.accountNumber}</Text>
+          <Text style={{ marginLeft: 8 }}>{record.accountName}</Text>
+        </span>
+      ),
+    },
+    {
+      title: 'Beloeb',
+      dataIndex: 'amount',
+      key: 'amount',
+      align: 'right' as const,
+      render: (value: number) => formatCurrency(value),
+    },
+    {
+      title: 'Moms',
+      dataIndex: 'vatCode',
+      key: 'vatCode',
+      render: (code: VATCode) => <Tag>{code}</Tag>,
+    },
+    {
+      title: '',
+      key: 'actions',
+      width: 50,
+      render: (_: unknown, __: unknown, index: number) => (
+        <Button
+          type="text"
+          danger
+          size="small"
+          icon={<DeleteOutlined />}
+          onClick={() => removeSplitLine(index)}
+        />
+      ),
+    },
+  ];
+
+  if (!bankTransaction) return null;
+
+  const canAddLine =
+    newLineAccountId &&
+    newLineAmount &&
+    newLineAmount > 0 &&
+    newLineAmount <= splitState.remainingAmount + 0.01;
+
+  const canSubmit =
+    preview?.isValid && splitState.lines.length > 0 && splitState.remainingAmount < 0.01;
+
+  return (
+    <Modal
+      title="Opdel transaktion"
+      open={isOpen}
+      onCancel={closeModal}
+      onOk={handleSubmit}
+      okText="Bogfoer opdeling"
+      cancelText="Annuller"
+      okButtonProps={{
+        disabled: !canSubmit || isSaving,
+        loading: isSaving,
+      }}
+      width={800}
+      destroyOnClose
+    >
+      {/* Bank transaction summary */}
+      <Card
+        size="small"
+        style={{
+          marginBottom: 16,
+          borderLeft: `3px solid ${isExpense ? accountingColors.debit : accountingColors.credit}`,
+        }}
+      >
+        <Row justify="space-between" align="middle">
+          <Col>
+            <Text>Banktransaktion:</Text>
+            <Text
+              strong
+              style={{
+                fontSize: 18,
+                marginLeft: 8,
+                color: isExpense ? accountingColors.debit : accountingColors.credit,
+              }}
+            >
+              {formatCurrency(bankTransaction.amount)}
+            </Text>
+            <Text style={{ marginLeft: 16 }}>{bankTransaction.description}</Text>
+          </Col>
+          <Col>
+            <Text type="secondary">{formatDateShort(bankTransaction.date)}</Text>
+          </Col>
+        </Row>
+      </Card>
+
+      {/* Remaining amount indicator */}
+      <Alert
+        type={splitState.remainingAmount < 0.01 ? 'success' : 'warning'}
+        message={
+          <span>
+            Restbeloeb:{' '}
+            <Text strong style={{ color: splitState.remainingAmount < 0.01 ? accountingColors.credit : accountingColors.warning }}>
+              {formatCurrency(splitState.remainingAmount)}
+            </Text>
+            {splitState.remainingAmount < 0.01 && ' - Fuld fordeling'}
+          </span>
+        }
+        style={{ marginBottom: 16 }}
+      />
+
+      {/* Existing split lines */}
+      {splitState.lines.length > 0 && (
+        <div style={{ marginBottom: 16 }}>
+          <Text strong style={{ display: 'block', marginBottom: 8 }}>
+            Fordeling:
+          </Text>
+          <Table
+            dataSource={splitState.lines.map((line, idx) => ({ ...line, key: idx }))}
+            columns={splitLinesColumns}
+            pagination={false}
+            size="small"
+          />
+        </div>
+      )}
+
+      {/* Add new line form */}
+      {splitState.remainingAmount > 0.01 && (
+        <Card size="small" title="Tilfoej linje" style={{ marginBottom: 16 }}>
+          <Row gutter={16}>
+            <Col span={10}>
+              <Form.Item label="Konto" style={{ marginBottom: 8 }}>
+                <AccountQuickPicker
+                  accounts={accountOptions}
+                  value={newLineAccountId}
+                  onChange={setNewLineAccountId}
+                  isExpense={isExpense}
+                  placeholder="Vaelg konto..."
+                />
+              </Form.Item>
+            </Col>
+            <Col span={6}>
+              <Form.Item label="Beloeb" style={{ marginBottom: 8 }}>
+                <Space.Compact style={{ width: '100%' }}>
+                  <InputNumber
+                    style={{ width: '100%' }}
+                    value={newLineAmount}
+                    onChange={(value) => setNewLineAmount(value)}
+                    min={0.01}
+                    max={splitState.remainingAmount}
+                    precision={2}
+                    placeholder="0,00"
+                    addonAfter="kr"
+                  />
+                  <Button onClick={handleFillRemaining} title="Udfyld resten">
+                    Rest
+                  </Button>
+                </Space.Compact>
+              </Form.Item>
+            </Col>
+            <Col span={5}>
+              <Form.Item label="Moms" style={{ marginBottom: 8 }}>
+                <VATCodePicker
+                  value={newLineVATCode}
+                  onChange={setNewLineVATCode}
+                  isExpense={isExpense}
+                />
+              </Form.Item>
+            </Col>
+            <Col span={3}>
+              <Form.Item label=" " style={{ marginBottom: 8 }}>
+                <Button
+                  type="primary"
+                  icon={<PlusOutlined />}
+                  onClick={handleAddLine}
+                  disabled={!canAddLine}
+                >
+                  Tilfoej
+                </Button>
+              </Form.Item>
+            </Col>
+          </Row>
+          <Row>
+            <Col span={24}>
+              <Form.Item label="Beskrivelse (valgfri)" style={{ marginBottom: 0 }}>
+                <Input
+                  value={newLineDescription}
+                  onChange={(e) => setNewLineDescription(e.target.value)}
+                  placeholder={bankTransaction.description}
+                />
+              </Form.Item>
+            </Col>
+          </Row>
+        </Card>
+      )}
+
+      {/* Preview */}
+      {preview && preview.lines.length > 0 && (
+        <>
+          <Divider style={{ margin: '16px 0' }} />
+          <div>
+            <div style={{ display: 'flex', alignItems: 'center', marginBottom: 8 }}>
+              <Text strong style={{ marginRight: 8 }}>
+                Forhaandsvisning af bilag:
+              </Text>
+              {preview.isValid ? (
+                <Tag color="success" icon={<CheckCircleOutlined />}>
+                  Balancerer
+                </Tag>
+              ) : (
+                <Tag color="error" icon={<WarningOutlined />}>
+                  Fejl
+                </Tag>
+              )}
+            </div>
+
+            {!preview.isValid && preview.validationMessage && (
+              <Alert
+                type="error"
+                message={preview.validationMessage}
+                style={{ marginBottom: 8 }}
+              />
+            )}
+
+            <Table
+              dataSource={preview.lines.map((line, idx) => ({ ...line, key: idx }))}
+              columns={previewColumns}
+              pagination={false}
+              size="small"
+              bordered
+              summary={() => {
+                const totalDebit = preview.lines.reduce((sum, l) => sum + l.debit, 0);
+                const totalCredit = preview.lines.reduce((sum, l) => sum + l.credit, 0);
+                return (
+                  <Table.Summary.Row>
+                    <Table.Summary.Cell index={0}>
+                      <Text strong>Total</Text>
+                    </Table.Summary.Cell>
+                    <Table.Summary.Cell index={1} align="right">
+                      <Text strong style={{ color: accountingColors.debit }}>
+                        {formatCurrency(totalDebit)}
+                      </Text>
+                    </Table.Summary.Cell>
+                    <Table.Summary.Cell index={2} align="right">
+                      <Text strong style={{ color: accountingColors.credit }}>
+                        {formatCurrency(totalCredit)}
+                      </Text>
+                    </Table.Summary.Cell>
+                  </Table.Summary.Row>
+                );
+              }}
+            />
+            <div style={{ marginTop: 8, textAlign: 'right' }}>
+              <Text type="secondary">
+                Debet = Kredit{' '}
+                {preview.isValid ? (
+                  <CheckCircleOutlined style={{ color: '#52c41a' }} />
+                ) : (
+                  <WarningOutlined style={{ color: '#ff4d4f' }} />
+                )}
+              </Text>
+            </div>
+          </div>
+        </>
+      )}
+    </Modal>
+  );
+}
+
+export default SplitBookModal;
--- a/frontend/src/components/simple-booking/index.ts
+++ b/frontend/src/components/simple-booking/index.ts
@ -0,0 +1,6 @@
+// Simple Booking Components - Export barrel
+
+export { BankTransactionCard, BankTransactionList } from './BankTransactionCard';
+export { AccountQuickPicker, VATCodePicker } from './AccountQuickPicker';
+export { QuickBookModal } from './QuickBookModal';
+export { SplitBookModal } from './SplitBookModal';
--- a/frontend/src/components/tables/DataTable.tsx
+++ b/frontend/src/components/tables/DataTable.tsx
@ -0,0 +1,302 @@
+import { Table, Button, Space, Tooltip, Empty, Typography } from 'antd';
+import { DownloadOutlined, ReloadOutlined } from '@ant-design/icons';
+import type { TableColumnType, TableProps } from 'antd';
+import { useDataTable } from '@/hooks/useDataTable';
+import { formatCurrency, formatDate, formatNumber } from '@/lib/formatters';
+import { getAmountClass } from '@/lib/formatters';
+import * as XLSX from 'xlsx';
+import { useCallback, useMemo } from 'react';
+
+const { Text } = Typography;
+
+export interface DataTableColumn<T> extends Omit<TableColumnType<T>, 'render' | 'dataIndex'> {
+  dataIndex: string | string[];
+  title: string;
+  sortable?: boolean;
+  filterable?: boolean;
+  render?: (value: unknown, record: T, index: number) => React.ReactNode;
+  columnType?: 'text' | 'number' | 'currency' | 'date' | 'boolean' | 'actions';
+  decimalPlaces?: number;
+  showSign?: boolean;
+  exportable?: boolean;
+}
+
+interface DataTableProps<T extends { id: string }> {
+  // Data source - either from hook or direct data
+  queryKey?: string[];
+  query?: string;
+  variables?: Record<string, unknown>;
+  dataPath?: string;
+  totalPath?: string;
+
+  // Or direct data
+  data?: T[];
+  loading?: boolean;
+
+  // Columns
+  columns: DataTableColumn<T>[];
+
+  // Pagination
+  pageSize?: number;
+  pageSizeOptions?: number[];
+
+  // Features
+  rowSelection?: 'single' | 'multiple' | false;
+  selectedRowKeys?: string[];
+  onSelectionChange?: (selectedRowKeys: string[], selectedRows: T[]) => void;
+  onRowClick?: (record: T) => void;
+  exportable?: boolean;
+  exportFilename?: string;
+  refreshable?: boolean;
+
+  // Row styling
+  rowClassName?: (record: T, index: number) => string;
+
+  // Actions
+  toolbarActions?: React.ReactNode;
+  toolbarTitle?: string;
+
+  // Empty state
+  emptyText?: string;
+
+  // Scroll
+  scroll?: TableProps<T>['scroll'];
+}
+
+export default function DataTable<T extends { id: string }>({
+  queryKey,
+  query,
+  variables,
+  dataPath,
+  totalPath,
+  data: directData,
+  loading: directLoading,
+  columns,
+  pageSize = 20,
+  rowSelection = false,
+  selectedRowKeys: externalSelectedKeys,
+  onSelectionChange,
+  onRowClick,
+  exportable = true,
+  exportFilename = 'export',
+  refreshable = true,
+  rowClassName,
+  toolbarActions,
+  toolbarTitle,
+  emptyText = 'Ingen data fundet',
+  scroll,
+}: DataTableProps<T>) {
+  // Use hook if query is provided, otherwise use direct data
+  const hookEnabled = !!(queryKey && query && dataPath && totalPath);
+
+  const {
+    data: hookData,
+    total,
+    isLoading: hookLoading,
+    pagination,
+    handleTableChange,
+    refetch,
+  } = useDataTable<T>({
+    queryKey: queryKey || [],
+    query: query || '',
+    variables,
+    dataPath: dataPath || '',
+    totalPath: totalPath || '',
+    pageSize,
+    enabled: hookEnabled,
+  });
+
+  const data = hookEnabled ? hookData : (directData || []);
+  const isLoading = hookEnabled ? hookLoading : (directLoading || false);
+
+  // Process columns with default renders based on columnType
+  const processedColumns = useMemo(() => {
+    return columns.map((col) => {
+      const processedCol: TableColumnType<T> = {
+        ...col,
+        sorter: col.sortable,
+      };
+
+      // Add default render based on columnType if no custom render
+      if (!col.render && col.columnType) {
+        switch (col.columnType) {
+          case 'currency':
+            processedCol.render = (value: unknown) => {
+              const numValue = typeof value === 'number' ? value : 0;
+              return (
+                <span className={`currency tabular-nums ${getAmountClass(numValue)}`}>
+                  {formatCurrency(numValue, {
+                    showSign: col.showSign,
+                    decimalPlaces: col.decimalPlaces,
+                  })}
+                </span>
+              );
+            };
+            processedCol.align = 'right';
+            break;
+
+          case 'number':
+            processedCol.render = (value: unknown) => {
+              const numValue = typeof value === 'number' ? value : 0;
+              return (
+                <span className="tabular-nums">
+                  {formatNumber(numValue, col.decimalPlaces)}
+                </span>
+              );
+            };
+            processedCol.align = 'right';
+            break;
+
+          case 'date':
+            processedCol.render = (value: unknown) => {
+              if (!value) return '-';
+              return formatDate(value as string);
+            };
+            break;
+
+          case 'boolean':
+            processedCol.render = (value: unknown) => {
+              return value ? 'Ja' : 'Nej';
+            };
+            break;
+        }
+      } else if (col.render) {
+        processedCol.render = col.render;
+      }
+
+      return processedCol;
+    });
+  }, [columns]);
+
+  // Row selection config
+  const rowSelectionConfig = useMemo(() => {
+    if (!rowSelection) return undefined;
+
+    return {
+      type: rowSelection === 'single' ? ('radio' as const) : ('checkbox' as const),
+      selectedRowKeys: externalSelectedKeys,
+      onChange: (keys: React.Key[], rows: T[]) => {
+        onSelectionChange?.(keys as string[], rows);
+      },
+    };
+  }, [rowSelection, externalSelectedKeys, onSelectionChange]);
+
+  // Export to Excel
+  const handleExport = useCallback(() => {
+    const exportData = data.map((record) => {
+      const row: Record<string, unknown> = {};
+      columns.forEach((col) => {
+        if (col.exportable !== false) {
+          const value = Array.isArray(col.dataIndex)
+            ? col.dataIndex.reduce(
+                (obj: unknown, k) =>
+                  obj && typeof obj === 'object' ? (obj as Record<string, unknown>)[k] : undefined,
+                record
+              )
+            : record[col.dataIndex as keyof T];
+
+          // Format value for export
+          if (col.columnType === 'currency' && typeof value === 'number') {
+            row[col.title] = value;
+          } else if (col.columnType === 'date' && value) {
+            row[col.title] = formatDate(value as string);
+          } else {
+            row[col.title] = value;
+          }
+        }
+      });
+      return row;
+    });
+
+    const worksheet = XLSX.utils.json_to_sheet(exportData);
+    const workbook = XLSX.utils.book_new();
+    XLSX.utils.book_append_sheet(workbook, worksheet, 'Data');
+
+    const filename = `${exportFilename}_${new Date().toISOString().split('T')[0]}.xlsx`;
+    XLSX.writeFile(workbook, filename);
+  }, [data, columns, exportFilename]);
+
+  // Row click handler
+  const onRow = useCallback(
+    (record: T) => ({
+      onClick: () => onRowClick?.(record),
+      style: onRowClick ? { cursor: 'pointer' } : undefined,
+    }),
+    [onRowClick]
+  );
+
+  return (
+    <div>
+      {/* Toolbar */}
+      {(toolbarTitle || toolbarActions || exportable || refreshable) && (
+        <div
+          style={{
+            display: 'flex',
+            justifyContent: 'space-between',
+            alignItems: 'center',
+            marginBottom: 16,
+          }}
+        >
+          <div>
+            {toolbarTitle && (
+              <Text strong style={{ fontSize: 16 }}>
+                {toolbarTitle}
+              </Text>
+            )}
+          </div>
+          <Space>
+            {toolbarActions}
+            {refreshable && hookEnabled && (
+              <Tooltip title="Genindlaes">
+                <Button
+                  icon={<ReloadOutlined />}
+                  onClick={() => refetch()}
+                  loading={isLoading}
+                />
+              </Tooltip>
+            )}
+            {exportable && data.length > 0 && (
+              <Tooltip title="Eksporter til Excel">
+                <Button
+                  icon={<DownloadOutlined />}
+                  onClick={handleExport}
+                >
+                  Eksporter
+                </Button>
+              </Tooltip>
+            )}
+          </Space>
+        </div>
+      )}
+
+      {/* Table */}
+      <Table<T>
+        columns={processedColumns}
+        dataSource={data}
+        rowKey="id"
+        loading={isLoading}
+        pagination={hookEnabled ? pagination : { pageSize, showSizeChanger: true }}
+        onChange={hookEnabled ? handleTableChange : undefined}
+        rowSelection={rowSelectionConfig}
+        onRow={onRow}
+        rowClassName={rowClassName}
+        scroll={scroll || { x: 'max-content' }}
+        size="small"
+        bordered
+        locale={{
+          emptyText: <Empty description={emptyText} />,
+        }}
+        style={{ width: '100%' }}
+      />
+
+      {/* Footer with total count */}
+      {hookEnabled && total > 0 && (
+        <div style={{ marginTop: 8, textAlign: 'right' }}>
+          <Text type="secondary">
+            Total: {total.toLocaleString('da-DK')} poster
+          </Text>
+        </div>
+      )}
+    </div>
+  );
+}
--- a/frontend/src/hooks/useCompany.ts
+++ b/frontend/src/hooks/useCompany.ts
@ -0,0 +1,70 @@
+import { useCompanyStore } from '@/stores/companyStore';
+import { useCallback } from 'react';
+import type { Company } from '@/types/accounting';
+
+/**
+ * Hook for accessing and managing the current company context
+ */
+export function useCompany() {
+  const activeCompany = useCompanyStore((state) => state.activeCompany);
+  const companies = useCompanyStore((state) => state.companies);
+  const isLoading = useCompanyStore((state) => state.isLoading);
+  const setActiveCompany = useCompanyStore((state) => state.setActiveCompany);
+
+  /**
+   * Get the current company ID or throw if not set
+   */
+  const requireCompanyId = useCallback((): string => {
+    if (!activeCompany) {
+      throw new Error('No company selected. Please select a company first.');
+    }
+    return activeCompany.id;
+  }, [activeCompany]);
+
+  /**
+   * Check if a company is selected
+   */
+  const hasCompany = !!activeCompany;
+
+  /**
+   * Switch to a different company
+   */
+  const switchCompany = useCallback(
+    (companyId: string) => {
+      const company = companies.find((c) => c.id === companyId);
+      if (company) {
+        setActiveCompany(company);
+      }
+    },
+    [companies, setActiveCompany]
+  );
+
+  /**
+   * Get company by ID
+   */
+  const getCompanyById = useCallback(
+    (id: string): Company | undefined => {
+      return companies.find((c) => c.id === id);
+    },
+    [companies]
+  );
+
+  return {
+    // Current company
+    company: activeCompany,
+    companyId: activeCompany?.id,
+    companyName: activeCompany?.name,
+
+    // All companies
+    companies,
+
+    // State
+    isLoading,
+    hasCompany,
+
+    // Actions
+    requireCompanyId,
+    switchCompany,
+    getCompanyById,
+  };
+}
--- a/frontend/src/hooks/useDataTable.ts
+++ b/frontend/src/hooks/useDataTable.ts
@ -0,0 +1,176 @@
+import { useState, useCallback, useMemo } from 'react';
+import { useQuery } from '@tanstack/react-query';
+import { graphqlClient } from '@/api/client';
+import type { SorterResult, TablePaginationConfig } from 'antd/es/table/interface';
+import type { FilterValue } from 'antd/es/table/interface';
+
+interface UseDataTableOptions {
+  queryKey: string[];
+  query: string;
+  variables?: Record<string, unknown>;
+  dataPath: string;
+  totalPath: string;
+  pageSize?: number;
+  enabled?: boolean;
+}
+
+interface UseDataTableReturn<T> {
+  data: T[];
+  total: number;
+  isLoading: boolean;
+  isError: boolean;
+  error: Error | null;
+  pagination: TablePaginationConfig;
+  handleTableChange: (
+    pagination: TablePaginationConfig,
+    filters: Record<string, FilterValue | null>,
+    sorter: SorterResult<T> | SorterResult<T>[]
+  ) => void;
+  refetch: () => void;
+  currentPage: number;
+  currentPageSize: number;
+  sortField: string | undefined;
+  sortOrder: 'asc' | 'desc' | undefined;
+  filters: Record<string, unknown>;
+  setFilters: (filters: Record<string, unknown>) => void;
+}
+
+function getNestedValue(obj: unknown, path: string): unknown {
+  return path.split('.').reduce((acc: unknown, part: string) => {
+    if (acc && typeof acc === 'object' && part in acc) {
+      return (acc as Record<string, unknown>)[part];
+    }
+    return undefined;
+  }, obj);
+}
+
+export function useDataTable<T extends { id: string }>({
+  queryKey,
+  query,
+  variables = {},
+  dataPath,
+  totalPath,
+  pageSize = 20,
+  enabled = true,
+}: UseDataTableOptions): UseDataTableReturn<T> {
+  const [currentPage, setCurrentPage] = useState(1);
+  const [currentPageSize, setCurrentPageSize] = useState(pageSize);
+  const [sortField, setSortField] = useState<string | undefined>();
+  const [sortOrder, setSortOrder] = useState<'asc' | 'desc' | undefined>();
+  const [filters, setFilters] = useState<Record<string, unknown>>({});
+
+  // Build query variables with pagination, sorting, and filtering
+  const queryVariables = useMemo(() => {
+    const vars: Record<string, unknown> = {
+      ...variables,
+      first: currentPageSize,
+      after: currentPage > 1 ? String((currentPage - 1) * currentPageSize) : undefined,
+    };
+
+    if (sortField && sortOrder) {
+      vars.sort = {
+        field: sortField,
+        direction: sortOrder.toUpperCase(),
+      };
+    }
+
+    if (Object.keys(filters).length > 0) {
+      vars.filter = filters;
+    }
+
+    return vars;
+  }, [variables, currentPage, currentPageSize, sortField, sortOrder, filters]);
+
+  // Execute GraphQL query
+  const { data: response, isLoading, isError, error, refetch } = useQuery({
+    queryKey: [...queryKey, queryVariables],
+    queryFn: async () => {
+      return graphqlClient.request(query, queryVariables);
+    },
+    enabled,
+  });
+
+  // Extract data and total from response
+  const data = useMemo(() => {
+    if (!response) return [];
+    const items = getNestedValue(response, dataPath);
+    return Array.isArray(items) ? items : [];
+  }, [response, dataPath]) as T[];
+
+  const total = useMemo(() => {
+    if (!response) return 0;
+    const count = getNestedValue(response, totalPath);
+    return typeof count === 'number' ? count : 0;
+  }, [response, totalPath]);
+
+  // Pagination config for Ant Design Table
+  const pagination: TablePaginationConfig = useMemo(
+    () => ({
+      current: currentPage,
+      pageSize: currentPageSize,
+      total,
+      showSizeChanger: true,
+      showQuickJumper: true,
+      pageSizeOptions: ['10', '20', '50', '100'],
+      showTotal: (total, range) =>
+        `${range[0]}-${range[1]} af ${total} poster`,
+    }),
+    [currentPage, currentPageSize, total]
+  );
+
+  // Handle table changes (pagination, sorting, filtering)
+  const handleTableChange = useCallback(
+    (
+      newPagination: TablePaginationConfig,
+      tableFilters: Record<string, FilterValue | null>,
+      sorter: SorterResult<T> | SorterResult<T>[]
+    ) => {
+      // Handle pagination
+      if (newPagination.current !== currentPage) {
+        setCurrentPage(newPagination.current || 1);
+      }
+      if (newPagination.pageSize !== currentPageSize) {
+        setCurrentPageSize(newPagination.pageSize || pageSize);
+        setCurrentPage(1); // Reset to first page on page size change
+      }
+
+      // Handle sorting
+      const singleSorter = Array.isArray(sorter) ? sorter[0] : sorter;
+      if (singleSorter.field && singleSorter.order) {
+        setSortField(String(singleSorter.field));
+        setSortOrder(singleSorter.order === 'ascend' ? 'asc' : 'desc');
+      } else {
+        setSortField(undefined);
+        setSortOrder(undefined);
+      }
+
+      // Handle filters from table columns
+      const newFilters: Record<string, unknown> = {};
+      Object.entries(tableFilters).forEach(([key, value]) => {
+        if (value !== null && value.length > 0) {
+          newFilters[key] = value.length === 1 ? value[0] : value;
+        }
+      });
+      // Merge with existing external filters
+      setFilters((prev) => ({ ...prev, ...newFilters }));
+    },
+    [currentPage, currentPageSize, pageSize]
+  );
+
+  return {
+    data,
+    total,
+    isLoading,
+    isError,
+    error: error as Error | null,
+    pagination,
+    handleTableChange,
+    refetch,
+    currentPage,
+    currentPageSize,
+    sortField,
+    sortOrder,
+    filters,
+    setFilters,
+  };
+}
--- a/frontend/src/hooks/usePeriod.ts
+++ b/frontend/src/hooks/usePeriod.ts
@ -0,0 +1,345 @@
+// Period Hook - React hook for period context and validation
+
+import { useMemo, useCallback } from 'react';
+import { usePeriodStore } from '@/stores/periodStore';
+import {
+  getPeriodForDate,
+  getPreviousPeriod,
+  getSamePeriodPreviousYear,
+  getYearToDateRange,
+  canPostToDate,
+  validatePeriodClose,
+} from '@/lib/periods';
+import type { AccountingPeriod, FiscalYear, PeriodStatus } from '@/types/periods';
+import type { Transaction } from '@/types/accounting';
+
+/**
+ * Hook for accessing period context in components
+ */
+export function usePeriodContext() {
+  const {
+    currentFiscalYear,
+    currentPeriod,
+    selectedPeriod,
+    selectedVATPeriod,
+    comparisonPeriod,
+    comparisonType,
+    periods,
+    fiscalYears,
+    vatPeriods,
+    isLoading,
+  } = usePeriodStore();
+
+  const effectivePeriod = selectedPeriod || currentPeriod;
+
+  return {
+    // Current context
+    currentFiscalYear,
+    currentPeriod,
+    selectedPeriod,
+    effectivePeriod,
+    selectedVATPeriod,
+    comparisonPeriod,
+    comparisonType,
+
+    // Lists
+    periods,
+    fiscalYears,
+    vatPeriods,
+
+    // Loading
+    isLoading,
+  };
+}
+
+/**
+ * Hook for period selection and navigation
+ */
+export function usePeriodSelector() {
+  const {
+    periods,
+    fiscalYears,
+    selectedPeriod,
+    setSelectedPeriod,
+    setComparisonPeriod,
+    clearComparison,
+  } = usePeriodStore();
+
+  const selectPeriod = useCallback(
+    (periodId: string) => {
+      const period = periods.find((p) => p.id === periodId);
+      if (period) {
+        setSelectedPeriod(period);
+      }
+    },
+    [periods, setSelectedPeriod]
+  );
+
+  const selectPreviousPeriod = useCallback(() => {
+    if (!selectedPeriod) return;
+    const previous = getPreviousPeriod(selectedPeriod, periods);
+    if (previous) {
+      setSelectedPeriod(previous);
+    }
+  }, [selectedPeriod, periods, setSelectedPeriod]);
+
+  const selectNextPeriod = useCallback(() => {
+    if (!selectedPeriod) return;
+    const currentIndex = periods.findIndex((p) => p.id === selectedPeriod.id);
+    if (currentIndex >= 0 && currentIndex < periods.length - 1) {
+      setSelectedPeriod(periods[currentIndex + 1]);
+    }
+  }, [selectedPeriod, periods, setSelectedPeriod]);
+
+  const enableComparison = useCallback(
+    (type: 'previous-period' | 'previous-year' | 'custom', customPeriod?: AccountingPeriod) => {
+      if (!selectedPeriod) return;
+
+      let comparisonPeriodData: AccountingPeriod | undefined;
+
+      if (type === 'previous-period') {
+        comparisonPeriodData = getPreviousPeriod(selectedPeriod, periods);
+      } else if (type === 'previous-year') {
+        comparisonPeriodData = getSamePeriodPreviousYear(selectedPeriod, periods);
+      } else if (type === 'custom' && customPeriod) {
+        comparisonPeriodData = customPeriod;
+      }
+
+      if (comparisonPeriodData) {
+        setComparisonPeriod(comparisonPeriodData, type);
+      }
+    },
+    [selectedPeriod, periods, setComparisonPeriod]
+  );
+
+  const disableComparison = useCallback(() => {
+    clearComparison();
+  }, [clearComparison]);
+
+  // Get periods grouped by fiscal year
+  const periodsByYear = useMemo(() => {
+    const grouped: Record<string, AccountingPeriod[]> = {};
+    for (const period of periods) {
+      if (!grouped[period.fiscalYearId]) {
+        grouped[period.fiscalYearId] = [];
+      }
+      grouped[period.fiscalYearId].push(period);
+    }
+    return grouped;
+  }, [periods]);
+
+  // Get open periods only
+  const openPeriods = useMemo(
+    () => periods.filter((p) => p.status === 'open'),
+    [periods]
+  );
+
+  return {
+    // State
+    selectedPeriod,
+    periods,
+    fiscalYears,
+    periodsByYear,
+    openPeriods,
+
+    // Actions
+    selectPeriod,
+    selectPreviousPeriod,
+    selectNextPeriod,
+    enableComparison,
+    disableComparison,
+  };
+}
+
+/**
+ * Hook for posting validation
+ */
+export function usePostingValidation() {
+  const { periods, periodSettings } = usePeriodStore();
+
+  const validatePostingDate = useCallback(
+    (date: string) => {
+      if (!periodSettings) {
+        // Default to strict validation if no settings
+        return canPostToDate(date, periods, {
+          preventPostingToClosedPeriods: true,
+          preventPostingToFuturePeriods: true,
+        });
+      }
+
+      return canPostToDate(date, periods, {
+        preventPostingToClosedPeriods: periodSettings.preventPostingToClosedPeriods,
+        preventPostingToFuturePeriods: periodSettings.preventPostingToFuturePeriods,
+      });
+    },
+    [periods, periodSettings]
+  );
+
+  const getPeriodStatus = useCallback(
+    (date: string): PeriodStatus | 'no-period' => {
+      const period = getPeriodForDate(date, periods);
+      return period?.status || 'no-period';
+    },
+    [periods]
+  );
+
+  const isDatePostable = useCallback(
+    (date: string): boolean => {
+      return validatePostingDate(date).allowed;
+    },
+    [validatePostingDate]
+  );
+
+  return {
+    validatePostingDate,
+    getPeriodStatus,
+    isDatePostable,
+  };
+}
+
+/**
+ * Hook for period management (closing, locking, etc.)
+ */
+export function usePeriodManagement() {
+  const {
+    periods,
+    closePeriod,
+    reopenPeriod,
+    lockPeriod,
+    updatePeriod,
+  } = usePeriodStore();
+
+  const canClosePeriod = useCallback(
+    (periodId: string, transactions: Transaction[]): { canClose: boolean; errors: string[]; warnings: string[] } => {
+      const period = periods.find((p) => p.id === periodId);
+      if (!period) {
+        return {
+          canClose: false,
+          errors: ['Periode ikke fundet'],
+          warnings: [],
+        };
+      }
+
+      const validation = validatePeriodClose(period, transactions, {
+        requireAllReconciled: true,
+      });
+
+      return {
+        canClose: validation.isValid,
+        errors: validation.errors.map((e) => e.messageDanish),
+        warnings: validation.warnings.map((w) => w.messageDanish),
+      };
+    },
+    [periods]
+  );
+
+  const closeAccountingPeriod = useCallback(
+    (periodId: string, userId: string) => {
+      closePeriod(periodId, userId);
+    },
+    [closePeriod]
+  );
+
+  const reopenAccountingPeriod = useCallback(
+    (periodId: string, userId: string) => {
+      reopenPeriod(periodId, userId);
+    },
+    [reopenPeriod]
+  );
+
+  const lockAccountingPeriod = useCallback(
+    (periodId: string, userId: string) => {
+      lockPeriod(periodId, userId);
+    },
+    [lockPeriod]
+  );
+
+  const getPeriodActions = useCallback(
+    (periodId: string) => {
+      const period = periods.find((p) => p.id === periodId);
+      if (!period) return { canClose: false, canReopen: false, canLock: false };
+
+      return {
+        canClose: period.status === 'open',
+        canReopen: period.status === 'closed',
+        canLock: period.status === 'closed',
+      };
+    },
+    [periods]
+  );
+
+  return {
+    canClosePeriod,
+    closeAccountingPeriod,
+    reopenAccountingPeriod,
+    lockAccountingPeriod,
+    getPeriodActions,
+  };
+}
+
+/**
+ * Hook for year-to-date calculations
+ */
+export function useYearToDate() {
+  const { currentFiscalYear, selectedPeriod, periods } = usePeriodStore();
+
+  const ytdRange = useMemo(() => {
+    if (!currentFiscalYear || !selectedPeriod) return null;
+    return getYearToDateRange(selectedPeriod, currentFiscalYear);
+  }, [currentFiscalYear, selectedPeriod]);
+
+  const ytdPeriods = useMemo(() => {
+    if (!currentFiscalYear || !selectedPeriod) return [];
+    return periods.filter((p) => {
+      if (p.fiscalYearId !== currentFiscalYear.id) return false;
+      return p.periodNumber <= selectedPeriod.periodNumber;
+    });
+  }, [currentFiscalYear, selectedPeriod, periods]);
+
+  return {
+    ytdRange,
+    ytdPeriods,
+    fiscalYear: currentFiscalYear,
+  };
+}
+
+/**
+ * Combined hook for common period operations
+ */
+export function usePeriod() {
+  const context = usePeriodContext();
+  const selector = usePeriodSelector();
+  const validation = usePostingValidation();
+  const management = usePeriodManagement();
+  const ytd = useYearToDate();
+
+  return {
+    // Context
+    ...context,
+
+    // Selector
+    selectPeriod: selector.selectPeriod,
+    selectPreviousPeriod: selector.selectPreviousPeriod,
+    selectNextPeriod: selector.selectNextPeriod,
+    enableComparison: selector.enableComparison,
+    disableComparison: selector.disableComparison,
+    periodsByYear: selector.periodsByYear,
+    openPeriods: selector.openPeriods,
+
+    // Validation
+    validatePostingDate: validation.validatePostingDate,
+    getPeriodStatus: validation.getPeriodStatus,
+    isDatePostable: validation.isDatePostable,
+
+    // Management
+    canClosePeriod: management.canClosePeriod,
+    closeAccountingPeriod: management.closeAccountingPeriod,
+    reopenAccountingPeriod: management.reopenAccountingPeriod,
+    lockAccountingPeriod: management.lockAccountingPeriod,
+    getPeriodActions: management.getPeriodActions,
+
+    // YTD
+    ytdRange: ytd.ytdRange,
+    ytdPeriods: ytd.ytdPeriods,
+  };
+}
--- a/frontend/src/lib/accounting.ts
+++ b/frontend/src/lib/accounting.ts
@ -0,0 +1,586 @@
+import type { TransactionLine, AccountType } from '@/types/accounting';
+
+/**
+ * Validate that total debits equal total credits (double-entry principle)
+ */
+export function validateDoubleEntry(lines: TransactionLine[]): {
+  valid: boolean;
+  totalDebit: number;
+  totalCredit: number;
+  difference: number;
+} {
+  const totalDebit = lines.reduce((sum, line) => sum + (line.debit || 0), 0);
+  const totalCredit = lines.reduce((sum, line) => sum + (line.credit || 0), 0);
+  const difference = Math.abs(totalDebit - totalCredit);
+
+  // Allow for small floating point differences (< 0.01)
+  const valid = difference < 0.01;
+
+  return {
+    valid,
+    totalDebit,
+    totalCredit,
+    difference,
+  };
+}
+
+/**
+ * Calculate line balance (debit - credit)
+ */
+export function calculateLineBalance(line: TransactionLine): number {
+  return (line.debit || 0) - (line.credit || 0);
+}
+
+/**
+ * Get account type from account number (Danish standard)
+ */
+export function getAccountTypeFromNumber(accountNumber: string): AccountType {
+  const num = parseInt(accountNumber, 10);
+
+  if (num >= 1000 && num < 2000) return 'asset';
+  if (num >= 2000 && num < 3000) return 'liability';
+  if (num >= 3000 && num < 4000) return 'equity';
+  if (num >= 4000 && num < 5000) return 'revenue';
+  if (num >= 5000 && num < 6000) return 'cogs';
+  if (num >= 6000 && num < 7000) return 'expense';
+  if (num >= 7000 && num < 8000) return 'personnel';
+  if (num >= 8000 && num < 9000) return 'financial';
+  if (num >= 9000 && num < 10000) return 'extraordinary';
+
+  return 'asset'; // Default fallback
+}
+
+/**
+ * Get Danish name for account type
+ */
+export function getAccountTypeName(type: AccountType): string {
+  const names: Record<AccountType, string> = {
+    asset: 'Aktiver',
+    liability: 'Passiver',
+    equity: 'Egenkapital',
+    revenue: 'Indtægter',
+    cogs: 'Vareforbrug',
+    expense: 'Driftsomkostninger',
+    personnel: 'Personaleomkostninger',
+    financial: 'Finansielle poster',
+    extraordinary: 'Ekstraordinære poster',
+  };
+  return names[type];
+}
+
+/**
+ * Get account number range for type
+ */
+export function getAccountNumberRange(type: AccountType): { min: number; max: number } {
+  const ranges: Record<AccountType, { min: number; max: number }> = {
+    asset: { min: 1000, max: 1999 },
+    liability: { min: 2000, max: 2999 },
+    equity: { min: 3000, max: 3999 },
+    revenue: { min: 4000, max: 4999 },
+    cogs: { min: 5000, max: 5999 },
+    expense: { min: 6000, max: 6999 },
+    personnel: { min: 7000, max: 7999 },
+    financial: { min: 8000, max: 8999 },
+    extraordinary: { min: 9000, max: 9999 },
+  };
+  return ranges[type];
+}
+
+/**
+ * Check if account is a balance sheet account (remains between periods)
+ */
+export function isBalanceSheetAccount(type: AccountType): boolean {
+  return ['asset', 'liability', 'equity'].includes(type);
+}
+
+/**
+ * Check if account is an income statement account (resets each period)
+ */
+export function isIncomeStatementAccount(type: AccountType): boolean {
+  return !isBalanceSheetAccount(type);
+}
+
+/**
+ * Calculate normal balance direction for account type
+ * Returns 'debit' or 'credit'
+ */
+export function getNormalBalance(type: AccountType): 'debit' | 'credit' {
+  // Asset and expense accounts normally have debit balances
+  // Liability, equity, and revenue accounts normally have credit balances
+  switch (type) {
+    case 'asset':
+    case 'cogs':
+    case 'expense':
+    case 'personnel':
+    case 'financial':
+    case 'extraordinary':
+      return 'debit';
+    case 'liability':
+    case 'equity':
+    case 'revenue':
+      return 'credit';
+  }
+}
+
+/**
+ * Danish VAT rates
+ */
+export const VAT_RATES = {
+  standard: 0.25, // 25% standard rate
+  reduced: 0, // No reduced rate in Denmark
+  zero: 0, // Zero-rated (export, etc.)
+} as const;
+
+// Note: VAT calculation functions (calculateVATFromGross, calculateVATFromNet)
+// are available from '@/lib/vatCodes' - use those to avoid duplication
+
+/**
+ * Standard Danish VAT codes
+ */
+export const VAT_CODES = {
+  S25: { code: 'S25', name: 'Udgående moms 25%', rate: 0.25, type: 'output' },
+  K25: { code: 'K25', name: 'Indgående moms 25%', rate: 0.25, type: 'input' },
+  E0: { code: 'E0', name: 'EU-varekøb 0%', rate: 0, type: 'eu' },
+  U0: { code: 'U0', name: 'Eksport 0%', rate: 0, type: 'export' },
+  NONE: { code: 'NONE', name: 'Ingen moms', rate: 0, type: 'none' },
+} as const;
+
+/**
+ * Generate next transaction number
+ */
+export function generateTransactionNumber(lastNumber: string | null, prefix: string = ''): string {
+  if (!lastNumber) {
+    return `${prefix}1`;
+  }
+  const numPart = lastNumber.replace(/\D/g, '');
+  const nextNum = parseInt(numPart, 10) + 1;
+  return `${prefix}${nextNum}`;
+}
+
+// =====================================================
+// AUTO-BOOKING FUNCTIONS (Simple Booking)
+// =====================================================
+
+import type { VATCode } from '@/types/vat';
+import { VAT_CODE_CONFIG, VAT_ACCOUNTS } from '@/lib/vatCodes';
+
+/**
+ * Bank transaction input for auto-booking
+ */
+export interface BankTransactionInput {
+  id: string;
+  date: string;
+  amount: number; // Negative = expense, Positive = income
+  description: string;
+  counterparty?: string;
+  bankAccountId: string;
+  bankAccountNumber: string;
+}
+
+/**
+ * Simple booking input (one bank transaction -> one contra account)
+ */
+export interface SimpleBookingInput {
+  bankTransaction: BankTransactionInput;
+  contraAccountId: string;
+  contraAccountNumber: string;
+  contraAccountName: string;
+  vatCode: VATCode;
+  description?: string;
+}
+
+/**
+ * Split booking line (for distributing one bank transaction to multiple accounts)
+ */
+export interface SplitBookingLine {
+  accountId: string;
+  accountNumber: string;
+  accountName: string;
+  amount: number; // Gross amount for this line
+  vatCode: VATCode;
+  description?: string;
+}
+
+/**
+ * Split booking input (one bank transaction -> multiple contra accounts)
+ */
+export interface SplitBookingInput {
+  bankTransaction: BankTransactionInput;
+  lines: SplitBookingLine[];
+}
+
+/**
+ * Generated transaction line with all details
+ */
+export interface GeneratedTransactionLine {
+  accountId: string;
+  accountNumber: string;
+  accountName: string;
+  description: string;
+  debit: number;
+  credit: number;
+  vatCode?: VATCode;
+  vatAmount?: number;
+}
+
+/**
+ * Generated transaction result
+ */
+export interface GeneratedTransaction {
+  date: string;
+  description: string;
+  lines: GeneratedTransactionLine[];
+  bankTransactionId: string;
+  isValid: boolean;
+  validationMessage?: string;
+}
+
+/**
+ * Generate simple double-entry from bank transaction
+ *
+ * For expense (negative bank amount, e.g., -15.000 kr husleje):
+ *   1. 6100 Husleje     Debit: 12.000 kr (net)
+ *   2. 5610 Moms (ind)  Debit:  3.000 kr (25% VAT)
+ *   3. 1000 Bank        Credit: 15.000 kr (gross)
+ *
+ * For income (positive bank amount, e.g., +15.625 kr salg):
+ *   1. 1000 Bank        Debit: 15.625 kr (gross)
+ *   2. 4000 Salg        Credit: 12.500 kr (net)
+ *   3. 5710 Moms (udg)  Credit:  3.125 kr (25% VAT)
+ */
+export function generateSimpleDoubleEntry(input: SimpleBookingInput): GeneratedTransaction {
+  const { bankTransaction, contraAccountId, contraAccountNumber, contraAccountName, vatCode } = input;
+  const description = input.description || bankTransaction.description;
+  const grossAmount = Math.abs(bankTransaction.amount);
+  const isExpense = bankTransaction.amount < 0;
+
+  const vatConfig = VAT_CODE_CONFIG[vatCode];
+  const lines: GeneratedTransactionLine[] = [];
+
+  // Calculate VAT if applicable
+  // Note: Round net first, then calculate VAT as gross - net
+  // This ensures net + vat = gross (no floating point drift)
+  let netAmount = grossAmount;
+  let vatAmount = 0;
+
+  if (vatConfig.rate > 0) {
+    const rawNet = grossAmount / (1 + vatConfig.rate);
+    netAmount = Math.round(rawNet * 100) / 100;
+    // Calculate VAT as difference to guarantee gross = net + vat
+    vatAmount = Math.round((grossAmount - netAmount) * 100) / 100;
+  }
+
+  if (isExpense) {
+    // Expense: Money leaving bank account
+    // Contra account (expense) gets debited with net amount
+    lines.push({
+      accountId: contraAccountId,
+      accountNumber: contraAccountNumber,
+      accountName: contraAccountName,
+      description,
+      debit: netAmount,
+      credit: 0,
+      vatCode,
+      vatAmount: vatConfig.rate > 0 ? vatAmount : undefined,
+    });
+
+    // VAT account (input VAT) gets debited if applicable
+    if (vatAmount > 0 && vatConfig.deductible) {
+      lines.push({
+        accountId: `vat-input-${vatCode}`,
+        accountNumber: VAT_ACCOUNTS.inputVAT,
+        accountName: 'Indgaaende moms',
+        description: `Moms: ${description}`,
+        debit: vatAmount,
+        credit: 0,
+      });
+    }
+
+    // For reverse charge (EU purchases), also credit output VAT
+    if (vatConfig.reverseCharge && vatAmount > 0) {
+      const outputVatAccount = vatCode === 'EU_VARE' ? VAT_ACCOUNTS.euVAT : VAT_ACCOUNTS.outputVAT;
+      lines.push({
+        accountId: `vat-output-${vatCode}`,
+        accountNumber: outputVatAccount,
+        accountName: vatCode === 'EU_VARE' ? 'EU-moms (erhvervelse)' : 'Udgaaende moms',
+        description: `Moms: ${description}`,
+        debit: 0,
+        credit: vatAmount,
+      });
+    }
+
+    // Bank account gets credited with gross amount
+    lines.push({
+      accountId: bankTransaction.bankAccountId,
+      accountNumber: bankTransaction.bankAccountNumber,
+      accountName: 'Bank',
+      description,
+      debit: 0,
+      credit: grossAmount,
+    });
+  } else {
+    // Income: Money entering bank account
+    // Bank account gets debited with gross amount
+    lines.push({
+      accountId: bankTransaction.bankAccountId,
+      accountNumber: bankTransaction.bankAccountNumber,
+      accountName: 'Bank',
+      description,
+      debit: grossAmount,
+      credit: 0,
+    });
+
+    // Contra account (revenue) gets credited with net amount
+    lines.push({
+      accountId: contraAccountId,
+      accountNumber: contraAccountNumber,
+      accountName: contraAccountName,
+      description,
+      debit: 0,
+      credit: netAmount,
+      vatCode,
+      vatAmount: vatConfig.rate > 0 ? vatAmount : undefined,
+    });
+
+    // VAT account (output VAT) gets credited if applicable
+    if (vatAmount > 0 && vatConfig.type === 'output') {
+      lines.push({
+        accountId: 'vat-output',
+        accountNumber: VAT_ACCOUNTS.outputVAT,
+        accountName: 'Udgaaende moms',
+        description: `Moms: ${description}`,
+        debit: 0,
+        credit: vatAmount,
+      });
+    }
+  }
+
+  // Validate double-entry
+  const validation = validateDoubleEntry(
+    lines.map((l) => ({ ...l, id: '', transactionId: '' }))
+  );
+
+  return {
+    date: bankTransaction.date,
+    description,
+    lines,
+    bankTransactionId: bankTransaction.id,
+    isValid: validation.valid,
+    validationMessage: validation.valid
+      ? undefined
+      : `Ubalance: Difference ${validation.difference.toFixed(2)} kr`,
+  };
+}
+
+/**
+ * Generate split double-entry from bank transaction
+ * Distributes one bank transaction to multiple contra accounts
+ */
+export function generateSplitDoubleEntry(input: SplitBookingInput): GeneratedTransaction {
+  const { bankTransaction, lines: splitLines } = input;
+  const grossAmount = Math.abs(bankTransaction.amount);
+  const isExpense = bankTransaction.amount < 0;
+
+  const generatedLines: GeneratedTransactionLine[] = [];
+
+  // Validate that split lines sum to bank transaction amount
+  const splitTotal = splitLines.reduce((sum, line) => sum + Math.abs(line.amount), 0);
+  if (Math.abs(splitTotal - grossAmount) > 0.01) {
+    return {
+      date: bankTransaction.date,
+      description: bankTransaction.description,
+      lines: [],
+      bankTransactionId: bankTransaction.id,
+      isValid: false,
+      validationMessage: `Split-beloeb (${splitTotal.toFixed(2)} kr) matcher ikke banktransaktion (${grossAmount.toFixed(2)} kr)`,
+    };
+  }
+
+  if (isExpense) {
+    // Process each split line for expenses
+    for (const splitLine of splitLines) {
+      const vatConfig = VAT_CODE_CONFIG[splitLine.vatCode];
+      const lineGross = Math.abs(splitLine.amount);
+      let lineNet = lineGross;
+      let lineVat = 0;
+
+      // Round net first, then calculate VAT as gross - net
+      // This ensures net + vat = gross (no floating point drift)
+      if (vatConfig.rate > 0) {
+        const rawNet = lineGross / (1 + vatConfig.rate);
+        lineNet = Math.round(rawNet * 100) / 100;
+        lineVat = Math.round((lineGross - lineNet) * 100) / 100;
+      }
+
+      const description = splitLine.description || bankTransaction.description;
+
+      // Contra account (expense) gets debited with net amount
+      generatedLines.push({
+        accountId: splitLine.accountId,
+        accountNumber: splitLine.accountNumber,
+        accountName: splitLine.accountName,
+        description,
+        debit: lineNet,
+        credit: 0,
+        vatCode: splitLine.vatCode,
+        vatAmount: lineVat > 0 ? lineVat : undefined,
+      });
+
+      // VAT account (input VAT) gets debited if applicable
+      if (lineVat > 0 && vatConfig.deductible) {
+        generatedLines.push({
+          accountId: `vat-input-${splitLine.vatCode}`,
+          accountNumber: VAT_ACCOUNTS.inputVAT,
+          accountName: 'Indgaaende moms',
+          description: `Moms: ${description}`,
+          debit: lineVat,
+          credit: 0,
+        });
+      }
+
+      // For reverse charge, also credit output VAT
+      if (vatConfig.reverseCharge && lineVat > 0) {
+        const outputVatAccount = splitLine.vatCode === 'EU_VARE' ? VAT_ACCOUNTS.euVAT : VAT_ACCOUNTS.outputVAT;
+        generatedLines.push({
+          accountId: `vat-output-${splitLine.vatCode}`,
+          accountNumber: outputVatAccount,
+          accountName: splitLine.vatCode === 'EU_VARE' ? 'EU-moms (erhvervelse)' : 'Udgaaende moms',
+          description: `Moms: ${description}`,
+          debit: 0,
+          credit: lineVat,
+        });
+      }
+    }
+
+    // Bank account gets credited with total gross amount
+    generatedLines.push({
+      accountId: bankTransaction.bankAccountId,
+      accountNumber: bankTransaction.bankAccountNumber,
+      accountName: 'Bank',
+      description: bankTransaction.description,
+      debit: 0,
+      credit: grossAmount,
+    });
+  } else {
+    // Income: Bank account gets debited first
+    generatedLines.push({
+      accountId: bankTransaction.bankAccountId,
+      accountNumber: bankTransaction.bankAccountNumber,
+      accountName: 'Bank',
+      description: bankTransaction.description,
+      debit: grossAmount,
+      credit: 0,
+    });
+
+    // Process each split line for income
+    for (const splitLine of splitLines) {
+      const vatConfig = VAT_CODE_CONFIG[splitLine.vatCode];
+      const lineGross = Math.abs(splitLine.amount);
+      let lineNet = lineGross;
+      let lineVat = 0;
+
+      // Round net first, then calculate VAT as gross - net
+      // This ensures net + vat = gross (no floating point drift)
+      if (vatConfig.rate > 0) {
+        const rawNet = lineGross / (1 + vatConfig.rate);
+        lineNet = Math.round(rawNet * 100) / 100;
+        lineVat = Math.round((lineGross - lineNet) * 100) / 100;
+      }
+
+      const description = splitLine.description || bankTransaction.description;
+
+      // Contra account (revenue) gets credited with net amount
+      generatedLines.push({
+        accountId: splitLine.accountId,
+        accountNumber: splitLine.accountNumber,
+        accountName: splitLine.accountName,
+        description,
+        debit: 0,
+        credit: lineNet,
+        vatCode: splitLine.vatCode,
+        vatAmount: lineVat > 0 ? lineVat : undefined,
+      });
+
+      // VAT account (output VAT) gets credited if applicable
+      if (lineVat > 0 && vatConfig.type === 'output') {
+        generatedLines.push({
+          accountId: 'vat-output',
+          accountNumber: VAT_ACCOUNTS.outputVAT,
+          accountName: 'Udgaaende moms',
+          description: `Moms: ${description}`,
+          debit: 0,
+          credit: lineVat,
+        });
+      }
+    }
+  }
+
+  // Validate double-entry
+  const validation = validateDoubleEntry(
+    generatedLines.map((l) => ({ ...l, id: '', transactionId: '' }))
+  );
+
+  return {
+    date: bankTransaction.date,
+    description: bankTransaction.description,
+    lines: generatedLines,
+    bankTransactionId: bankTransaction.id,
+    isValid: validation.valid,
+    validationMessage: validation.valid
+      ? undefined
+      : `Ubalance: Difference ${validation.difference.toFixed(2)} kr`,
+  };
+}
+
+/**
+ * Get suggested VAT code based on account type
+ */
+export function getSuggestedVATCode(accountNumber: string, isExpense: boolean): VATCode {
+  const accountType = getAccountTypeFromNumber(accountNumber);
+
+  // Expenses typically use input VAT (K25)
+  if (isExpense) {
+    // Some expense types are typically VAT-exempt
+    if (accountType === 'financial') return 'NONE';
+    if (accountType === 'personnel') return 'NONE';
+    return 'K25';
+  }
+
+  // Revenue typically uses output VAT (S25)
+  if (accountType === 'revenue') {
+    return 'S25';
+  }
+
+  return 'NONE';
+}
+
+/**
+ * Consolidate generated lines by account
+ * Combines lines with the same account to reduce line count
+ */
+export function consolidateTransactionLines(
+  lines: GeneratedTransactionLine[]
+): GeneratedTransactionLine[] {
+  const consolidated: Map<string, GeneratedTransactionLine> = new Map();
+
+  for (const line of lines) {
+    const key = line.accountNumber;
+    const existing = consolidated.get(key);
+
+    if (existing) {
+      existing.debit += line.debit;
+      existing.credit += line.credit;
+      // Append description if different
+      if (line.description && !existing.description.includes(line.description)) {
+        existing.description = `${existing.description}; ${line.description}`;
+      }
+    } else {
+      consolidated.set(key, { ...line });
+    }
+  }
+
+  // Filter out lines with zero amounts
+  return Array.from(consolidated.values()).filter(
+    (line) => line.debit > 0 || line.credit > 0
+  );
+}
--- a/frontend/src/lib/fiscalYear.ts
+++ b/frontend/src/lib/fiscalYear.ts
@ -0,0 +1,611 @@
+// Fiscal Year Utilities for Danish Accounting (Regnskabsår)
+
+import dayjs from 'dayjs';
+import type {
+  FiscalYear,
+  AccountingPeriod,
+  ClosingEntryType,
+} from '@/types/periods';
+import type { Account, Transaction } from '@/types/accounting';
+
+// =====================================================
+// FISCAL YEAR BOUNDARIES
+// =====================================================
+
+/**
+ * Calculate fiscal year boundaries for a given date
+ * Based on company's fiscal year start month
+ */
+export function getFiscalYearBoundaries(
+  date: string,
+  fiscalYearStartMonth: number
+): { startDate: string; endDate: string; name: string } {
+  const d = dayjs(date);
+  const year = d.year();
+  const month = d.month() + 1; // dayjs months are 0-indexed
+
+  let startYear: number;
+  let endYear: number;
+
+  if (fiscalYearStartMonth === 1) {
+    // Calendar year (January start)
+    startYear = year;
+    endYear = year;
+  } else {
+    // Non-calendar fiscal year (e.g., July start)
+    if (month >= fiscalYearStartMonth) {
+      // We're in the first part of a fiscal year that spans two calendar years
+      startYear = year;
+      endYear = year + 1;
+    } else {
+      // We're in the second part of a fiscal year
+      startYear = year - 1;
+      endYear = year;
+    }
+  }
+
+  const startDate = dayjs()
+    .year(startYear)
+    .month(fiscalYearStartMonth - 1)
+    .date(1)
+    .format('YYYY-MM-DD');
+
+  const endDate = dayjs()
+    .year(endYear)
+    .month(fiscalYearStartMonth - 1)
+    .subtract(1, 'day')
+    .endOf('month')
+    .format('YYYY-MM-DD');
+
+  // Name format: "2025" for calendar year, "2024/2025" for split year
+  const name = startYear === endYear ? `${startYear}` : `${startYear}/${endYear}`;
+
+  return { startDate, endDate, name };
+}
+
+/**
+ * Check if a date falls within a fiscal year
+ */
+export function isDateInFiscalYear(date: string, fiscalYear: FiscalYear): boolean {
+  const d = dayjs(date);
+  const start = dayjs(fiscalYear.startDate);
+  const end = dayjs(fiscalYear.endDate);
+
+  return (d.isAfter(start) || d.isSame(start, 'day')) &&
+         (d.isBefore(end) || d.isSame(end, 'day'));
+}
+
+/**
+ * Find the fiscal year that contains a given date
+ */
+export function findFiscalYearForDate(
+  date: string,
+  fiscalYears: FiscalYear[]
+): FiscalYear | undefined {
+  return fiscalYears.find((fy) => isDateInFiscalYear(date, fy));
+}
+
+/**
+ * Check if a date is outside all existing fiscal years
+ */
+export function isDateOutsideFiscalYears(
+  date: string,
+  fiscalYears: FiscalYear[]
+): boolean {
+  return !findFiscalYearForDate(date, fiscalYears);
+}
+
+// =====================================================
+// FISCAL YEAR CREATION
+// =====================================================
+
+/**
+ * Generate a new fiscal year object
+ */
+export function createFiscalYearObject(
+  companyId: string,
+  startDate: string,
+  endDate: string,
+  name: string
+): Omit<FiscalYear, 'id' | 'createdAt' | 'updatedAt'> {
+  return {
+    companyId,
+    name,
+    startDate,
+    endDate,
+    status: 'open',
+    openingBalancePosted: false,
+  };
+}
+
+/**
+ * Generate fiscal year for a specific date
+ */
+export function generateFiscalYearForDate(
+  date: string,
+  companyId: string,
+  fiscalYearStartMonth: number
+): Omit<FiscalYear, 'id' | 'createdAt' | 'updatedAt'> {
+  const { startDate, endDate, name } = getFiscalYearBoundaries(date, fiscalYearStartMonth);
+  return createFiscalYearObject(companyId, startDate, endDate, name);
+}
+
+// =====================================================
+// OPENING BALANCE CALCULATION
+// =====================================================
+
+/**
+ * Account balance at end of fiscal year
+ */
+export interface AccountClosingBalance {
+  accountId: string;
+  accountNumber: string;
+  accountName: string;
+  accountType: Account['type'];
+  closingBalance: number;
+}
+
+/**
+ * Opening balance for new fiscal year
+ */
+export interface OpeningBalance {
+  accountId: string;
+  accountNumber: string;
+  accountName: string;
+  accountType: Account['type'];
+  openingBalance: number;
+}
+
+/**
+ * Calculate closing balances for all accounts in a fiscal year
+ */
+export function calculateClosingBalances(
+  fiscalYear: FiscalYear,
+  accounts: Account[],
+  transactions: Transaction[]
+): AccountClosingBalance[] {
+  // Filter transactions to this fiscal year
+  const yearTransactions = transactions.filter((tx) => {
+    if (tx.isVoided) return false;
+    return isDateInFiscalYear(tx.date, fiscalYear);
+  });
+
+  // Calculate balance per account
+  const balances = new Map<string, number>();
+
+  for (const tx of yearTransactions) {
+    for (const line of tx.lines) {
+      const current = balances.get(line.accountId) || 0;
+      const change = (line.debit || 0) - (line.credit || 0);
+      balances.set(line.accountId, current + change);
+    }
+  }
+
+  // Build closing balance records
+  return accounts.map((account) => ({
+    accountId: account.id,
+    accountNumber: account.accountNumber,
+    accountName: account.name,
+    accountType: account.type,
+    closingBalance: balances.get(account.id) || 0,
+  }));
+}
+
+/**
+ * Calculate opening balances for a new fiscal year
+ * Based on previous year's closing balances
+ *
+ * Rules:
+ * - Balance sheet accounts (asset, liability, equity): carry forward
+ * - Income statement accounts (revenue, expense, cogs, personnel, financial, extraordinary): zero
+ * - Result is transferred to retained earnings (handled separately in closing entries)
+ */
+export function calculateOpeningBalances(
+  closingBalances: AccountClosingBalance[]
+): OpeningBalance[] {
+  const balanceSheetTypes: Account['type'][] = ['asset', 'liability', 'equity'];
+
+  return closingBalances
+    .filter((cb) => balanceSheetTypes.includes(cb.accountType))
+    .map((cb) => ({
+      accountId: cb.accountId,
+      accountNumber: cb.accountNumber,
+      accountName: cb.accountName,
+      accountType: cb.accountType,
+      openingBalance: cb.closingBalance,
+    }));
+}
+
+// =====================================================
+// YEAR-END CLOSING ENTRIES
+// =====================================================
+
+/**
+ * Closing summary for display before closing
+ */
+export interface FiscalYearClosingSummary {
+  fiscalYearId: string;
+  totalRevenue: number;
+  totalExpenses: number;
+  netResult: number;
+  unreconciledCount: number;
+  openPeriodsCount: number;
+  canClose: boolean;
+  warnings: string[];
+  errors: string[];
+}
+
+/**
+ * Calculate closing summary for a fiscal year
+ */
+export function calculateClosingSummary(
+  fiscalYear: FiscalYear,
+  periods: AccountingPeriod[],
+  closingBalances: AccountClosingBalance[],
+  transactions: Transaction[]
+): FiscalYearClosingSummary {
+  const warnings: string[] = [];
+  const errors: string[] = [];
+
+  // Calculate totals from closing balances
+  const revenueTypes: Account['type'][] = ['revenue'];
+  const expenseTypes: Account['type'][] = ['cogs', 'expense', 'personnel', 'financial', 'extraordinary'];
+
+  // Revenue is typically credit (negative balance in our debit-credit system)
+  const totalRevenue = Math.abs(
+    closingBalances
+      .filter((cb) => revenueTypes.includes(cb.accountType))
+      .reduce((sum, cb) => sum + cb.closingBalance, 0)
+  );
+
+  // Expenses are typically debit (positive balance)
+  const totalExpenses = closingBalances
+    .filter((cb) => expenseTypes.includes(cb.accountType))
+    .reduce((sum, cb) => sum + cb.closingBalance, 0);
+
+  const netResult = totalRevenue - totalExpenses;
+
+  // Check for unreconciled transactions
+  const yearTransactions = transactions.filter(
+    (tx) => !tx.isVoided && isDateInFiscalYear(tx.date, fiscalYear)
+  );
+  const unreconciledCount = yearTransactions.filter((tx) => !tx.isReconciled).length;
+  if (unreconciledCount > 0) {
+    warnings.push(
+      `${unreconciledCount} transaktioner er ikke afstemt`
+    );
+  }
+
+  // Check for open periods
+  const yearPeriods = periods.filter((p) => p.fiscalYearId === fiscalYear.id);
+  const openPeriodsCount = yearPeriods.filter((p) => p.status === 'open').length;
+  if (openPeriodsCount > 0) {
+    warnings.push(
+      `${openPeriodsCount} perioder er stadig åbne`
+    );
+  }
+
+  // Check if fiscal year is already closed/locked
+  if (fiscalYear.status === 'locked') {
+    errors.push('Regnskabsåret er allerede låst');
+  }
+
+  const canClose = errors.length === 0;
+
+  return {
+    fiscalYearId: fiscalYear.id,
+    totalRevenue,
+    totalExpenses,
+    netResult,
+    unreconciledCount,
+    openPeriodsCount,
+    canClose,
+    warnings,
+    errors,
+  };
+}
+
+/**
+ * Generate closing entries for year-end
+ */
+export interface GeneratedClosingEntry {
+  type: ClosingEntryType;
+  description: string;
+  descriptionDanish: string;
+  lines: Array<{
+    accountId: string;
+    accountNumber: string;
+    accountName: string;
+    debit: number;
+    credit: number;
+  }>;
+  totalAmount: number;
+}
+
+/**
+ * Generate year-end closing entries
+ */
+export function generateClosingEntries(
+  fiscalYear: FiscalYear,
+  closingBalances: AccountClosingBalance[],
+  resultAccountId: string,
+  resultAccountNumber: string,
+  resultAccountName: string
+): GeneratedClosingEntry[] {
+  const entries: GeneratedClosingEntry[] = [];
+
+  const revenueTypes: Account['type'][] = ['revenue'];
+  const expenseTypes: Account['type'][] = ['cogs', 'expense', 'personnel', 'financial', 'extraordinary'];
+
+  // 1. Close revenue accounts to result account
+  const revenueBalances = closingBalances.filter(
+    (cb) => revenueTypes.includes(cb.accountType) && cb.closingBalance !== 0
+  );
+
+  if (revenueBalances.length > 0) {
+    const revenueLines = revenueBalances.map((cb) => ({
+      accountId: cb.accountId,
+      accountNumber: cb.accountNumber,
+      accountName: cb.accountName,
+      // Revenue typically has credit balance (negative), so we debit to close
+      debit: cb.closingBalance < 0 ? Math.abs(cb.closingBalance) : 0,
+      credit: cb.closingBalance > 0 ? cb.closingBalance : 0,
+    }));
+
+    const totalRevenue = revenueBalances.reduce((sum, cb) => sum + cb.closingBalance, 0);
+
+    // Counter entry to result account
+    revenueLines.push({
+      accountId: resultAccountId,
+      accountNumber: resultAccountNumber,
+      accountName: resultAccountName,
+      debit: totalRevenue > 0 ? totalRevenue : 0,
+      credit: totalRevenue < 0 ? Math.abs(totalRevenue) : 0,
+    });
+
+    entries.push({
+      type: 'revenue-close',
+      description: `Close revenue accounts for fiscal year ${fiscalYear.name}`,
+      descriptionDanish: `Luk indtægtskonti for regnskabsår ${fiscalYear.name}`,
+      lines: revenueLines,
+      totalAmount: Math.abs(totalRevenue),
+    });
+  }
+
+  // 2. Close expense accounts to result account
+  const expenseBalances = closingBalances.filter(
+    (cb) => expenseTypes.includes(cb.accountType) && cb.closingBalance !== 0
+  );
+
+  if (expenseBalances.length > 0) {
+    const expenseLines = expenseBalances.map((cb) => ({
+      accountId: cb.accountId,
+      accountNumber: cb.accountNumber,
+      accountName: cb.accountName,
+      // Expenses typically have debit balance (positive), so we credit to close
+      debit: cb.closingBalance < 0 ? Math.abs(cb.closingBalance) : 0,
+      credit: cb.closingBalance > 0 ? cb.closingBalance : 0,
+    }));
+
+    const totalExpenses = expenseBalances.reduce((sum, cb) => sum + cb.closingBalance, 0);
+
+    // Counter entry to result account
+    expenseLines.push({
+      accountId: resultAccountId,
+      accountNumber: resultAccountNumber,
+      accountName: resultAccountName,
+      debit: totalExpenses < 0 ? Math.abs(totalExpenses) : 0,
+      credit: totalExpenses > 0 ? totalExpenses : 0,
+    });
+
+    entries.push({
+      type: 'expense-close',
+      description: `Close expense accounts for fiscal year ${fiscalYear.name}`,
+      descriptionDanish: `Luk udgiftskonti for regnskabsår ${fiscalYear.name}`,
+      lines: expenseLines,
+      totalAmount: Math.abs(totalExpenses),
+    });
+  }
+
+  return entries;
+}
+
+// =====================================================
+// VALIDATION
+// =====================================================
+
+/**
+ * Validation result for fiscal year operations
+ */
+export interface FiscalYearValidation {
+  isValid: boolean;
+  canProceed: boolean;
+  errors: Array<{ code: string; message: string; messageDanish: string }>;
+  warnings: Array<{ code: string; message: string; messageDanish: string }>;
+}
+
+/**
+ * Validate if a fiscal year can be closed
+ */
+export function validateFiscalYearClose(
+  fiscalYear: FiscalYear,
+  periods: AccountingPeriod[]
+): FiscalYearValidation {
+  const errors: FiscalYearValidation['errors'] = [];
+  const warnings: FiscalYearValidation['warnings'] = [];
+
+  // Check if already locked
+  if (fiscalYear.status === 'locked') {
+    errors.push({
+      code: 'ALREADY_LOCKED',
+      message: 'Fiscal year is already locked',
+      messageDanish: 'Regnskabsåret er allerede låst',
+    });
+  }
+
+  // Check for open periods
+  const yearPeriods = periods.filter((p) => p.fiscalYearId === fiscalYear.id);
+  const openPeriods = yearPeriods.filter((p) => p.status === 'open');
+
+  if (openPeriods.length > 0) {
+    warnings.push({
+      code: 'OPEN_PERIODS',
+      message: `${openPeriods.length} accounting period(s) are still open`,
+      messageDanish: `${openPeriods.length} regnskabsperiode(r) er stadig åbne`,
+    });
+  }
+
+  // Check for future periods
+  const futurePeriods = yearPeriods.filter((p) => p.status === 'future');
+  if (futurePeriods.length > 0) {
+    warnings.push({
+      code: 'FUTURE_PERIODS',
+      message: `${futurePeriods.length} accounting period(s) have not started yet`,
+      messageDanish: `${futurePeriods.length} regnskabsperiode(r) er ikke startet endnu`,
+    });
+  }
+
+  return {
+    isValid: errors.length === 0,
+    canProceed: errors.length === 0,
+    errors,
+    warnings,
+  };
+}
+
+/**
+ * Validate if a new fiscal year can be created
+ */
+export function validateFiscalYearCreation(
+  newYear: Pick<FiscalYear, 'startDate' | 'endDate'>,
+  existingYears: FiscalYear[]
+): FiscalYearValidation {
+  const errors: FiscalYearValidation['errors'] = [];
+  const warnings: FiscalYearValidation['warnings'] = [];
+
+  const newStart = dayjs(newYear.startDate);
+  const newEnd = dayjs(newYear.endDate);
+
+  // Check for overlapping fiscal years
+  for (const existing of existingYears) {
+    const existingStart = dayjs(existing.startDate);
+    const existingEnd = dayjs(existing.endDate);
+
+    // Use strict inequality - two fiscal years that touch at the boundary
+    // (one ends on date X, other starts on date X) are allowed
+    const overlaps = newStart.isBefore(existingEnd) && newEnd.isAfter(existingStart);
+
+    if (overlaps) {
+      errors.push({
+        code: 'OVERLAP',
+        message: `Overlaps with existing fiscal year ${existing.name}`,
+        messageDanish: `Overlapper med eksisterende regnskabsår ${existing.name}`,
+      });
+    }
+  }
+
+  // Check if fiscal year length is reasonable (300-400 days typically)
+  const daysDiff = newEnd.diff(newStart, 'day');
+  if (daysDiff < 300) {
+    warnings.push({
+      code: 'SHORT_YEAR',
+      message: `Fiscal year is only ${daysDiff} days`,
+      messageDanish: `Regnskabsåret er kun ${daysDiff} dage`,
+    });
+  }
+  if (daysDiff > 400) {
+    warnings.push({
+      code: 'LONG_YEAR',
+      message: `Fiscal year is ${daysDiff} days (typically should be ~365)`,
+      messageDanish: `Regnskabsåret er ${daysDiff} dage (normalt omkring 365)`,
+    });
+  }
+
+  return {
+    isValid: errors.length === 0,
+    canProceed: errors.length === 0,
+    errors,
+    warnings,
+  };
+}
+
+// =====================================================
+// HELPERS
+// =====================================================
+
+/**
+ * Get previous fiscal year
+ */
+export function getPreviousFiscalYear(
+  fiscalYear: FiscalYear,
+  allYears: FiscalYear[]
+): FiscalYear | undefined {
+  const sorted = [...allYears].sort(
+    (a, b) => new Date(b.startDate).getTime() - new Date(a.startDate).getTime()
+  );
+
+  const currentIndex = sorted.findIndex((y) => y.id === fiscalYear.id);
+  if (currentIndex === -1 || currentIndex === sorted.length - 1) {
+    return undefined;
+  }
+
+  return sorted[currentIndex + 1];
+}
+
+/**
+ * Get next fiscal year
+ */
+export function getNextFiscalYear(
+  fiscalYear: FiscalYear,
+  allYears: FiscalYear[]
+): FiscalYear | undefined {
+  const sorted = [...allYears].sort(
+    (a, b) => new Date(b.startDate).getTime() - new Date(a.startDate).getTime()
+  );
+
+  const currentIndex = sorted.findIndex((y) => y.id === fiscalYear.id);
+  if (currentIndex === -1 || currentIndex === 0) {
+    return undefined;
+  }
+
+  return sorted[currentIndex - 1];
+}
+
+/**
+ * Format fiscal year for display
+ */
+export function formatFiscalYear(fiscalYear: FiscalYear): string {
+  return `Regnskabsår ${fiscalYear.name}`;
+}
+
+/**
+ * Get fiscal year status color
+ */
+export function getFiscalYearStatusColor(status: FiscalYear['status']): string {
+  switch (status) {
+    case 'open':
+      return 'green';
+    case 'closed':
+      return 'orange';
+    case 'locked':
+      return 'red';
+    default:
+      return 'default';
+  }
+}
+
+/**
+ * Get fiscal year status label in Danish
+ */
+export function getFiscalYearStatusLabel(status: FiscalYear['status']): string {
+  switch (status) {
+    case 'open':
+      return 'Åben';
+    case 'closed':
+      return 'Lukket';
+    case 'locked':
+      return 'Låst';
+    default:
+      return status;
+  }
+}
--- a/frontend/src/lib/formatters.ts
+++ b/frontend/src/lib/formatters.ts
@ -0,0 +1,167 @@
+import dayjs from 'dayjs';
+import 'dayjs/locale/da';
+
+// Set Danish locale as default
+dayjs.locale('da');
+
+/**
+ * Format a number as Danish currency (kr.)
+ */
+export function formatCurrency(
+  value: number,
+  options: {
+    showSign?: boolean;
+    decimalPlaces?: number;
+    currency?: string;
+  } = {}
+): string {
+  const { showSign = false, decimalPlaces = 2, currency = 'kr.' } = options;
+
+  const absValue = Math.abs(value);
+  const formattedNumber = absValue.toLocaleString('da-DK', {
+    minimumFractionDigits: decimalPlaces,
+    maximumFractionDigits: decimalPlaces,
+  });
+
+  let result = `${formattedNumber} ${currency}`;
+
+  if (showSign && value !== 0) {
+    result = value > 0 ? `+${result}` : `-${result}`;
+  } else if (value < 0) {
+    result = `-${result}`;
+  }
+
+  return result;
+}
+
+/**
+ * Format a number with Danish locale (1.234,56)
+ */
+export function formatNumber(
+  value: number,
+  decimalPlaces: number = 2
+): string {
+  return value.toLocaleString('da-DK', {
+    minimumFractionDigits: decimalPlaces,
+    maximumFractionDigits: decimalPlaces,
+  });
+}
+
+/**
+ * Format a date in Danish format
+ */
+export function formatDate(
+  date: string | Date,
+  format: string = 'DD/MM/YYYY'
+): string {
+  return dayjs(date).format(format);
+}
+
+/**
+ * Format a date with time
+ */
+export function formatDateTime(
+  date: string | Date,
+  format: string = 'DD/MM/YYYY HH:mm'
+): string {
+  return dayjs(date).format(format);
+}
+
+/**
+ * Format a date in short Danish format (e.g., "15. jan 2025")
+ */
+export function formatDateShort(date: string | Date): string {
+  return dayjs(date).format('D. MMM YYYY');
+}
+
+/**
+ * Format a date in long Danish format (e.g., "15. januar 2025")
+ */
+export function formatDateLong(date: string | Date): string {
+  return dayjs(date).format('D. MMMM YYYY');
+}
+
+/**
+ * Format a date for API requests (ISO format)
+ */
+export function formatDateISO(date: string | Date): string {
+  return dayjs(date).format('YYYY-MM-DD');
+}
+
+/**
+ * Format period (month/year)
+ */
+export function formatPeriod(date: string | Date): string {
+  return dayjs(date).format('MMMM YYYY');
+}
+
+/**
+ * Format account number (e.g., "1000" -> "1000")
+ */
+export function formatAccountNumber(accountNumber: string): string {
+  // Already formatted, but could add padding if needed
+  return accountNumber.padStart(4, '0');
+}
+
+/**
+ * Format CVR number (e.g., "12345678" -> "12 34 56 78")
+ */
+export function formatCVR(cvr: string): string {
+  const cleaned = cvr.replace(/\D/g, '');
+  if (cleaned.length !== 8) return cvr;
+  return `${cleaned.slice(0, 2)} ${cleaned.slice(2, 4)} ${cleaned.slice(4, 6)} ${cleaned.slice(6, 8)}`;
+}
+
+/**
+ * Get CSS class for amount (positive/negative/zero)
+ */
+export function getAmountClass(value: number): string {
+  if (value > 0) return 'amount-positive';
+  if (value < 0) return 'amount-negative';
+  return 'amount-zero';
+}
+
+/**
+ * Parse a Danish formatted number string to number
+ */
+export function parseDanishNumber(value: string): number {
+  // Replace Danish thousand separator (.) with nothing
+  // Replace Danish decimal separator (,) with dot
+  const normalized = value
+    .replace(/\./g, '')
+    .replace(',', '.')
+    .replace(/[^\d.-]/g, '');
+  return parseFloat(normalized) || 0;
+}
+
+/**
+ * Format transaction number
+ */
+export function formatTransactionNumber(number: string | number): string {
+  return `#${number}`;
+}
+
+/**
+ * Format percentage
+ */
+export function formatPercentage(value: number, decimalPlaces: number = 1): string {
+  return `${formatNumber(value * 100, decimalPlaces)}%`;
+}
+
+/**
+ * Truncate text with ellipsis
+ */
+export function truncateText(text: string, maxLength: number): string {
+  if (text.length <= maxLength) return text;
+  return `${text.slice(0, maxLength - 3)}...`;
+}
+
+/**
+ * Format file size
+ */
+export function formatFileSize(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+  if (bytes < 1024 * 1024 * 1024) return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+  return `${(bytes / (1024 * 1024 * 1024)).toFixed(1)} GB`;
+}
--- a/frontend/src/lib/periods.ts
+++ b/frontend/src/lib/periods.ts
@ -0,0 +1,541 @@
+// Period Calculation Utilities for Danish Accounting
+
+import dayjs from 'dayjs';
+import type {
+  FiscalYear,
+  AccountingPeriod,
+  VATPeriod,
+  PeriodFrequency,
+  PeriodStatus,
+  PeriodValidationResult,
+  DANISH_MONTHS,
+  DANISH_MONTHS_SHORT,
+} from '@/types/periods';
+import type { VATPeriodicitet } from '@/types/periods';
+import type { Transaction } from '@/types/accounting';
+
+// =====================================================
+// PERIOD GENERATION
+// =====================================================
+
+/**
+ * Generate accounting periods for a fiscal year
+ */
+export function generateAccountingPeriods(
+  fiscalYear: FiscalYear,
+  frequency: PeriodFrequency
+): Omit<AccountingPeriod, 'id' | 'createdAt' | 'updatedAt'>[] {
+  const periods: Omit<AccountingPeriod, 'id' | 'createdAt' | 'updatedAt'>[] = [];
+  const startDate = dayjs(fiscalYear.startDate);
+  const endDate = dayjs(fiscalYear.endDate);
+
+  let periodCount: number;
+  let monthsPerPeriod: number;
+
+  switch (frequency) {
+    case 'monthly':
+      periodCount = 12;
+      monthsPerPeriod = 1;
+      break;
+    case 'quarterly':
+      periodCount = 4;
+      monthsPerPeriod = 3;
+      break;
+    case 'half-yearly':
+      periodCount = 2;
+      monthsPerPeriod = 6;
+      break;
+    case 'yearly':
+      periodCount = 1;
+      monthsPerPeriod = 12;
+      break;
+  }
+
+  for (let i = 0; i < periodCount; i++) {
+    const periodStart = startDate.add(i * monthsPerPeriod, 'month');
+    const periodEnd = periodStart.add(monthsPerPeriod, 'month').subtract(1, 'day');
+
+    // Make sure we don't exceed fiscal year end
+    const actualEnd = periodEnd.isAfter(endDate) ? endDate : periodEnd;
+
+    const { name, shortName } = getPeriodDisplayName(
+      periodStart.format('YYYY-MM-DD'),
+      actualEnd.format('YYYY-MM-DD'),
+      frequency
+    );
+
+    const status = determinePeriodStatus(
+      periodStart.format('YYYY-MM-DD'),
+      actualEnd.format('YYYY-MM-DD')
+    );
+
+    periods.push({
+      companyId: fiscalYear.companyId,
+      fiscalYearId: fiscalYear.id,
+      periodNumber: i + 1,
+      name,
+      shortName,
+      startDate: periodStart.format('YYYY-MM-DD'),
+      endDate: actualEnd.format('YYYY-MM-DD'),
+      status,
+    });
+  }
+
+  return periods;
+}
+
+/**
+ * Generate VAT periods for a year
+ */
+export function generateVATPeriods(
+  companyId: string,
+  periodicitet: VATPeriodicitet,
+  year: number
+): Omit<VATPeriod, 'id' | 'createdAt' | 'updatedAt'>[] {
+  const periods: Omit<VATPeriod, 'id' | 'createdAt' | 'updatedAt'>[] = [];
+
+  let periodCount: number;
+  let monthsPerPeriod: number;
+  let deadlineDays: number;
+
+  switch (periodicitet) {
+    case 'monthly':
+      periodCount = 12;
+      monthsPerPeriod = 1;
+      deadlineDays = 25;
+      break;
+    case 'quarterly':
+      periodCount = 4;
+      monthsPerPeriod = 3;
+      deadlineDays = 40;
+      break;
+    case 'half-yearly':
+      periodCount = 2;
+      monthsPerPeriod = 6;
+      deadlineDays = 60;
+      break;
+    case 'yearly':
+      periodCount = 1;
+      monthsPerPeriod = 12;
+      deadlineDays = 90;
+      break;
+  }
+
+  for (let i = 0; i < periodCount; i++) {
+    const startMonth = i * monthsPerPeriod + 1;
+    const periodStart = dayjs(`${year}-${String(startMonth).padStart(2, '0')}-01`);
+    const periodEnd = periodStart.add(monthsPerPeriod, 'month').subtract(1, 'day');
+    const deadline = periodEnd.add(deadlineDays, 'day');
+
+    const name = getVATPeriodName(periodicitet, year, i + 1);
+    const status = determineVATPeriodStatus(
+      periodStart.format('YYYY-MM-DD'),
+      periodEnd.format('YYYY-MM-DD')
+    );
+
+    periods.push({
+      companyId,
+      periodicitet,
+      year,
+      periodNumber: i + 1,
+      name,
+      startDate: periodStart.format('YYYY-MM-DD'),
+      endDate: periodEnd.format('YYYY-MM-DD'),
+      deadline: deadline.format('YYYY-MM-DD'),
+      status,
+    });
+  }
+
+  return periods;
+}
+
+// =====================================================
+// PERIOD NAMING
+// =====================================================
+
+/**
+ * Get period display name based on frequency
+ */
+export function getPeriodDisplayName(
+  startDate: string,
+  endDate: string,
+  frequency: PeriodFrequency
+): { name: string; shortName: string } {
+  const start = dayjs(startDate);
+  const MONTHS = [
+    'Januar', 'Februar', 'Marts', 'April', 'Maj', 'Juni',
+    'Juli', 'August', 'September', 'Oktober', 'November', 'December'
+  ];
+  const MONTHS_SHORT = [
+    'Jan', 'Feb', 'Mar', 'Apr', 'Maj', 'Jun',
+    'Jul', 'Aug', 'Sep', 'Okt', 'Nov', 'Dec'
+  ];
+
+  switch (frequency) {
+    case 'monthly':
+      return {
+        name: `${MONTHS[start.month()]} ${start.year()}`,
+        shortName: `${MONTHS_SHORT[start.month()]} ${start.year()}`,
+      };
+    case 'quarterly': {
+      const quarter = Math.floor(start.month() / 3) + 1;
+      return {
+        name: `Q${quarter} ${start.year()}`,
+        shortName: `Q${quarter}`,
+      };
+    }
+    case 'half-yearly': {
+      const half = start.month() < 6 ? 1 : 2;
+      return {
+        name: `H${half} ${start.year()}`,
+        shortName: `H${half}`,
+      };
+    }
+    case 'yearly':
+      return {
+        name: `${start.year()}`,
+        shortName: `${start.year()}`,
+      };
+  }
+}
+
+/**
+ * Get VAT period name
+ */
+export function getVATPeriodName(
+  periodicitet: VATPeriodicitet,
+  year: number,
+  periodNumber: number
+): string {
+  const MONTHS = [
+    'Januar', 'Februar', 'Marts', 'April', 'Maj', 'Juni',
+    'Juli', 'August', 'September', 'Oktober', 'November', 'December'
+  ];
+
+  switch (periodicitet) {
+    case 'monthly':
+      return `${MONTHS[periodNumber - 1]} ${year}`;
+    case 'quarterly':
+      return `Q${periodNumber} ${year}`;
+    case 'half-yearly':
+      return `H${periodNumber} ${year}`;
+    case 'yearly':
+      return `${year}`;
+  }
+}
+
+/**
+ * Get Danish month name
+ */
+export function getDanishMonthName(month: number): string {
+  const MONTHS = [
+    'Januar', 'Februar', 'Marts', 'April', 'Maj', 'Juni',
+    'Juli', 'August', 'September', 'Oktober', 'November', 'December'
+  ];
+  return MONTHS[month - 1] || '';
+}
+
+// =====================================================
+// PERIOD STATUS
+// =====================================================
+
+/**
+ * Determine period status based on dates
+ */
+export function determinePeriodStatus(
+  startDate: string,
+  endDate: string
+): PeriodStatus {
+  const now = dayjs();
+  const start = dayjs(startDate);
+  const end = dayjs(endDate);
+
+  if (now.isBefore(start)) return 'future';
+  if (now.isBefore(end) || now.isSame(end, 'day')) return 'open';
+  return 'closed';
+}
+
+/**
+ * Determine VAT period status
+ */
+export function determineVATPeriodStatus(
+  startDate: string,
+  endDate: string
+): 'future' | 'open' | 'closed' {
+  const now = dayjs();
+  const start = dayjs(startDate);
+  const end = dayjs(endDate);
+
+  if (now.isBefore(start)) return 'future';
+  if (now.isBefore(end) || now.isSame(end, 'day')) return 'open';
+  return 'closed';
+}
+
+// =====================================================
+// PERIOD LOOKUP
+// =====================================================
+
+/**
+ * Get period containing a specific date
+ */
+export function getPeriodForDate(
+  date: string,
+  periods: AccountingPeriod[]
+): AccountingPeriod | undefined {
+  const targetDate = dayjs(date);
+
+  return periods.find((period) => {
+    const start = dayjs(period.startDate);
+    const end = dayjs(period.endDate);
+    return (
+      (targetDate.isAfter(start) || targetDate.isSame(start, 'day')) &&
+      (targetDate.isBefore(end) || targetDate.isSame(end, 'day'))
+    );
+  });
+}
+
+/**
+ * Get previous period (for comparison)
+ */
+export function getPreviousPeriod(
+  currentPeriod: AccountingPeriod,
+  allPeriods: AccountingPeriod[]
+): AccountingPeriod | undefined {
+  const currentStart = dayjs(currentPeriod.startDate);
+
+  return allPeriods.find((period) => {
+    const periodEnd = dayjs(period.endDate);
+    // Previous period ends the day before current period starts
+    return periodEnd.add(1, 'day').isSame(currentStart, 'day');
+  });
+}
+
+/**
+ * Get same period from previous year
+ */
+export function getSamePeriodPreviousYear(
+  currentPeriod: AccountingPeriod,
+  allPeriods: AccountingPeriod[]
+): AccountingPeriod | undefined {
+  const currentStart = dayjs(currentPeriod.startDate);
+  const previousYearStart = currentStart.subtract(1, 'year');
+
+  return allPeriods.find((period) => {
+    const periodStart = dayjs(period.startDate);
+    return (
+      periodStart.month() === previousYearStart.month() &&
+      periodStart.year() === previousYearStart.year() &&
+      period.periodNumber === currentPeriod.periodNumber
+    );
+  });
+}
+
+/**
+ * Calculate year-to-date range
+ */
+export function getYearToDateRange(
+  currentPeriod: AccountingPeriod,
+  fiscalYear: FiscalYear
+): { startDate: string; endDate: string } {
+  return {
+    startDate: fiscalYear.startDate,
+    endDate: currentPeriod.endDate,
+  };
+}
+
+// =====================================================
+// PERIOD VALIDATION
+// =====================================================
+
+/**
+ * Check if a date can be posted to based on period status
+ */
+export function canPostToDate(
+  date: string,
+  periods: AccountingPeriod[],
+  settings: {
+    preventPostingToClosedPeriods: boolean;
+    preventPostingToFuturePeriods: boolean;
+  }
+): { allowed: boolean; reason?: string; reasonDanish?: string } {
+  const period = getPeriodForDate(date, periods);
+
+  if (!period) {
+    return {
+      allowed: false,
+      reason: 'No period found for this date',
+      reasonDanish: 'Ingen periode fundet for denne dato',
+    };
+  }
+
+  if (period.status === 'locked') {
+    return {
+      allowed: false,
+      reason: 'Period is locked',
+      reasonDanish: 'Perioden er laast',
+    };
+  }
+
+  if (period.status === 'closed' && settings.preventPostingToClosedPeriods) {
+    return {
+      allowed: false,
+      reason: 'Period is closed',
+      reasonDanish: 'Perioden er lukket',
+    };
+  }
+
+  if (period.status === 'future' && settings.preventPostingToFuturePeriods) {
+    return {
+      allowed: false,
+      reason: 'Cannot post to future periods',
+      reasonDanish: 'Kan ikke bogfoere i fremtidige perioder',
+    };
+  }
+
+  return { allowed: true };
+}
+
+/**
+ * Validate if period can be closed
+ */
+export function validatePeriodClose(
+  period: AccountingPeriod,
+  transactions: Transaction[],
+  options?: {
+    requireAllReconciled?: boolean;
+  }
+): PeriodValidationResult {
+  const errors: PeriodValidationResult['errors'] = [];
+  const warnings: PeriodValidationResult['warnings'] = [];
+
+  // Check if period is already locked
+  if (period.status === 'locked') {
+    errors.push({
+      code: 'PERIOD_LOCKED',
+      message: 'Period is already locked and cannot be modified',
+      messageDanish: 'Perioden er allerede laast og kan ikke aendres',
+    });
+  }
+
+  // Check for unreconciled transactions
+  if (options?.requireAllReconciled) {
+    const unreconciledCount = transactions.filter((tx) => !tx.isReconciled).length;
+    if (unreconciledCount > 0) {
+      warnings.push({
+        code: 'UNRECONCILED_TRANSACTIONS',
+        message: `${unreconciledCount} transactions are not reconciled`,
+        messageDanish: `${unreconciledCount} transaktioner er ikke afstemt`,
+      });
+    }
+  }
+
+  return {
+    isValid: errors.length === 0,
+    canPost: period.status !== 'locked',
+    errors,
+    warnings,
+  };
+}
+
+// =====================================================
+// VAT PERIOD HELPERS
+// =====================================================
+
+/**
+ * Calculate VAT deadlines based on SKAT rules
+ */
+export function calculateVATDeadline(
+  periodEndDate: string,
+  periodicitet: VATPeriodicitet
+): { deadline: string; paymentDeadline: string } {
+  const endDate = dayjs(periodEndDate);
+  let deadlineDays: number;
+
+  switch (periodicitet) {
+    case 'monthly':
+      deadlineDays = 25;
+      break;
+    case 'quarterly':
+      deadlineDays = 40;
+      break;
+    case 'half-yearly':
+      deadlineDays = 60;
+      break;
+    case 'yearly':
+      deadlineDays = 90;
+      break;
+  }
+
+  const deadline = endDate.add(deadlineDays, 'day');
+  // Payment deadline is typically same as reporting deadline
+  const paymentDeadline = deadline;
+
+  return {
+    deadline: deadline.format('YYYY-MM-DD'),
+    paymentDeadline: paymentDeadline.format('YYYY-MM-DD'),
+  };
+}
+
+/**
+ * Format period for SKAT export
+ */
+export function formatPeriodForSKAT(
+  periodicitet: VATPeriodicitet,
+  year: number,
+  periodNumber: number
+): string {
+  switch (periodicitet) {
+    case 'monthly':
+      return `${year}${String(periodNumber).padStart(2, '0')}`;
+    case 'quarterly':
+      return `${year}Q${periodNumber}`;
+    case 'half-yearly':
+      return `${year}H${periodNumber}`;
+    case 'yearly':
+      return `${year}`;
+  }
+}
+
+// =====================================================
+// FISCAL YEAR HELPERS
+// =====================================================
+
+/**
+ * Create a new fiscal year
+ */
+export function createFiscalYear(
+  companyId: string,
+  startMonth: number,
+  year: number
+): Omit<FiscalYear, 'id' | 'createdAt' | 'updatedAt'> {
+  const startDate = dayjs(`${year}-${String(startMonth).padStart(2, '0')}-01`);
+  const endDate = startDate.add(1, 'year').subtract(1, 'day');
+
+  // Determine fiscal year name
+  const name = startMonth === 1
+    ? `${year}`
+    : `${year}/${year + 1}`;
+
+  return {
+    companyId,
+    name,
+    startDate: startDate.format('YYYY-MM-DD'),
+    endDate: endDate.format('YYYY-MM-DD'),
+    status: 'open',
+    openingBalancePosted: false,
+  };
+}
+
+/**
+ * Check if a date is within a fiscal year
+ */
+export function isDateInFiscalYear(date: string, fiscalYear: FiscalYear): boolean {
+  const targetDate = dayjs(date);
+  const start = dayjs(fiscalYear.startDate);
+  const end = dayjs(fiscalYear.endDate);
+
+  return (
+    (targetDate.isAfter(start) || targetDate.isSame(start, 'day')) &&
+    (targetDate.isBefore(end) || targetDate.isSame(end, 'day'))
+  );
+}
--- a/frontend/src/lib/vatCalculation.ts
+++ b/frontend/src/lib/vatCalculation.ts
@ -0,0 +1,419 @@
+// VAT Calculation Engine for Danish SKAT Compliance
+
+import dayjs from 'dayjs';
+import type {
+  VATBoxId,
+  BasisBoxId,
+  VATReportPeriod,
+  VATTransactionLine,
+  VATCalculationResult,
+  VATCalculationWarning,
+  VATCalculationError,
+  CalculatedVATBox,
+  VATReport,
+  SKATExportCSV,
+  SKATExportXML,
+} from '@/types/vat';
+import type { Transaction, TransactionLine, Company } from '@/types/accounting';
+import {
+  VAT_CODE_CONFIG,
+  SKAT_VAT_BOXES,
+  calculateVATFromGross,
+  isValidVATCode,
+} from './vatCodes';
+
+// =====================================================
+// MAIN CALCULATION FUNCTION
+// =====================================================
+
+/**
+ * Calculate VAT report from transactions
+ */
+export function calculateVATReport(
+  transactions: Transaction[],
+  period: VATReportPeriod
+): VATCalculationResult {
+  const warnings: VATCalculationWarning[] = [];
+  const errors: VATCalculationError[] = [];
+  const vatLines: VATTransactionLine[] = [];
+
+  // Initialize box accumulators
+  const boxes: Record<VATBoxId | BasisBoxId, {
+    amount: number;
+    transactionCount: number;
+    transactionIds: string[];
+  }> = {
+    A: { amount: 0, transactionCount: 0, transactionIds: [] },
+    B: { amount: 0, transactionCount: 0, transactionIds: [] },
+    C: { amount: 0, transactionCount: 0, transactionIds: [] },
+    D: { amount: 0, transactionCount: 0, transactionIds: [] },
+    '1': { amount: 0, transactionCount: 0, transactionIds: [] },
+    '2': { amount: 0, transactionCount: 0, transactionIds: [] },
+    '3': { amount: 0, transactionCount: 0, transactionIds: [] },
+    '4': { amount: 0, transactionCount: 0, transactionIds: [] },
+  };
+
+  // Process each transaction
+  for (const transaction of transactions) {
+    // Check transaction is within period
+    const txDate = dayjs(transaction.date);
+    const periodStart = dayjs(period.startDate);
+    const periodEnd = dayjs(period.endDate);
+
+    if (txDate.isBefore(periodStart) || txDate.isAfter(periodEnd)) {
+      warnings.push({
+        type: 'period_mismatch',
+        message: `Transaction ${transaction.transactionNumber} is outside the period`,
+        messageDanish: `Transaktion ${transaction.transactionNumber} er uden for perioden`,
+        transactionId: transaction.id,
+        severity: 'medium',
+      });
+      continue;
+    }
+
+    // Skip voided transactions
+    if (transaction.isVoided) continue;
+
+    // Process each line
+    for (const line of transaction.lines) {
+      const vatLine = processTransactionLine(transaction, line, warnings, errors);
+      if (vatLine) {
+        vatLines.push(vatLine);
+
+        // Add to appropriate boxes
+        const codeConfig = VAT_CODE_CONFIG[vatLine.vatCode];
+
+        if (codeConfig.affectsBoxes.vatBox) {
+          const boxId = codeConfig.affectsBoxes.vatBox;
+          boxes[boxId].amount += vatLine.vatAmount;
+          if (!boxes[boxId].transactionIds.includes(transaction.id)) {
+            boxes[boxId].transactionIds.push(transaction.id);
+            boxes[boxId].transactionCount++;
+          }
+        }
+
+        if (codeConfig.affectsBoxes.basisBox) {
+          const basisId = codeConfig.affectsBoxes.basisBox;
+          boxes[basisId].amount += Math.abs(vatLine.netAmount);
+          if (!boxes[basisId].transactionIds.includes(transaction.id)) {
+            boxes[basisId].transactionIds.push(transaction.id);
+            boxes[basisId].transactionCount++;
+          }
+        }
+
+        // For reverse charge, also add to input VAT (Box B) as deductible
+        if (codeConfig.reverseCharge && codeConfig.deductible) {
+          boxes.B.amount += vatLine.vatAmount;
+          if (!boxes.B.transactionIds.includes(transaction.id)) {
+            boxes.B.transactionIds.push(transaction.id);
+            boxes.B.transactionCount++;
+          }
+        }
+      }
+    }
+  }
+
+  // Calculate summary
+  const totalOutputVAT = boxes.A.amount + boxes.C.amount + boxes.D.amount;
+  const totalInputVAT = boxes.B.amount;
+  const netVAT = totalOutputVAT - totalInputVAT;
+
+  return {
+    period,
+    transactions: vatLines,
+    boxes,
+    summary: {
+      totalOutputVAT: roundToOre(totalOutputVAT),
+      totalInputVAT: roundToOre(totalInputVAT),
+      netVAT: roundToOre(netVAT),
+    },
+    warnings,
+    errors,
+  };
+}
+
+/**
+ * Process a single transaction line for VAT
+ */
+function processTransactionLine(
+  transaction: Transaction,
+  line: TransactionLine,
+  _warnings: VATCalculationWarning[],
+  errors: VATCalculationError[]
+): VATTransactionLine | null {
+  // Skip lines without VAT code
+  if (!line.vatCode) {
+    return null;
+  }
+
+  // Validate VAT code instead of using type assertion
+  if (!isValidVATCode(line.vatCode)) {
+    errors.push({
+      type: 'invalid_vat_code',
+      message: `Invalid VAT code: ${line.vatCode}`,
+      messageDanish: `Ugyldig momskode: ${line.vatCode}`,
+      transactionId: transaction.id,
+    });
+    return null;
+  }
+
+  const vatCode = line.vatCode;
+
+  // Skip lines without VAT relevance
+  if (vatCode === 'NONE') {
+    return null;
+  }
+
+  const codeConfig = VAT_CODE_CONFIG[vatCode];
+
+  // Calculate net amount (amount without VAT)
+  const grossAmount = line.debit > 0 ? line.debit : line.credit;
+  let netAmount: number;
+  let vatAmount: number;
+
+  if (line.vatAmount !== undefined && line.vatAmount !== null) {
+    // VAT amount explicitly provided
+    vatAmount = line.vatAmount;
+    netAmount = grossAmount - vatAmount;
+  } else if (codeConfig.rate > 0) {
+    // Calculate VAT from gross amount
+    const calculated = calculateVATFromGross(grossAmount, codeConfig.rate);
+    netAmount = calculated.net;
+    vatAmount = calculated.vat;
+  } else {
+    // Zero-rated or exempt
+    netAmount = grossAmount;
+    vatAmount = 0;
+  }
+
+  // Determine sign based on transaction type
+  const isOutput = codeConfig.type === 'output';
+  const isInput = codeConfig.type === 'input' || codeConfig.type === 'reverse_charge';
+
+  // For output VAT (sales), credit increases VAT liability
+  // For input VAT (purchases), debit increases VAT deduction
+  if (isOutput && line.debit > 0) {
+    // This is a reversal/return of sales
+    netAmount = -netAmount;
+    vatAmount = -vatAmount;
+  } else if (isInput && line.credit > 0) {
+    // This is a reversal/return of purchases
+    netAmount = -netAmount;
+    vatAmount = -vatAmount;
+  }
+
+  return {
+    transactionId: transaction.id,
+    transactionLineId: line.id,
+    transactionNumber: transaction.transactionNumber,
+    transactionDate: transaction.date,
+    accountId: line.accountId,
+    accountNumber: line.account?.accountNumber || '',
+    accountName: line.account?.name || '',
+    description: line.description || transaction.description,
+    debit: line.debit,
+    credit: line.credit,
+    netAmount: roundToOre(netAmount),
+    vatCode,
+    vatAmount: roundToOre(Math.abs(vatAmount)),
+    vatRate: codeConfig.rate,
+    isReverseCharge: codeConfig.reverseCharge,
+  };
+}
+
+// =====================================================
+// EXPORT FUNCTIONS
+// =====================================================
+
+/**
+ * Generate SKAT CSV export data
+ */
+export function generateSKATCSV(
+  calculation: VATCalculationResult,
+  company: Company
+): SKATExportCSV {
+  return {
+    cvr: company.cvr.replace(/\s/g, ''),
+    periode: formatPeriodForSKATExport(calculation.period),
+    rubrikA: Math.round(calculation.boxes.A.amount),
+    rubrikB: Math.round(calculation.boxes.B.amount),
+    rubrikC: Math.round(calculation.boxes.C.amount),
+    rubrikD: Math.round(calculation.boxes.D.amount),
+    felt1: Math.round(calculation.boxes['1'].amount),
+    felt2: Math.round(calculation.boxes['2'].amount),
+    felt3: Math.round(calculation.boxes['3'].amount),
+    felt4: Math.round(calculation.boxes['4'].amount),
+  };
+}
+
+/**
+ * Generate SKAT XML export data
+ */
+export function generateSKATXML(
+  calculation: VATCalculationResult,
+  company: Company
+): SKATExportXML {
+  return {
+    version: '1.0',
+    cvr: company.cvr.replace(/\s/g, ''),
+    periodeStart: calculation.period.startDate,
+    periodeSlut: calculation.period.endDate,
+    angivelse: {
+      salgsmoms: roundToOre(calculation.boxes.A.amount),
+      koebsmoms: roundToOre(calculation.boxes.B.amount),
+      euVarekoebMoms: roundToOre(calculation.boxes.C.amount),
+      ydelseskoebMoms: roundToOre(calculation.boxes.D.amount),
+      salgMedMoms: roundToOre(calculation.boxes['1'].amount),
+      salgUdenMoms: roundToOre(calculation.boxes['2'].amount),
+      euVarekoeb: roundToOre(calculation.boxes['3'].amount),
+      ydelseskoeb: roundToOre(calculation.boxes['4'].amount),
+    },
+  };
+}
+
+/**
+ * Export VAT report as CSV string
+ */
+export function exportVATReportCSV(
+  calculation: VATCalculationResult,
+  company: Company
+): string {
+  const data = generateSKATCSV(calculation, company);
+
+  const header = 'CVR;Periode;RubrikA;RubrikB;RubrikC;RubrikD;Felt1;Felt2;Felt3;Felt4';
+  const row = `${data.cvr};${data.periode};${data.rubrikA};${data.rubrikB};${data.rubrikC};${data.rubrikD};${data.felt1};${data.felt2};${data.felt3};${data.felt4}`;
+
+  return `${header}\n${row}`;
+}
+
+/**
+ * Export VAT report as XML string
+ */
+export function exportVATReportXML(
+  calculation: VATCalculationResult,
+  company: Company
+): string {
+  const data = generateSKATXML(calculation, company);
+
+  return `<?xml version="1.0" encoding="UTF-8"?>
+<Momsangivelse version="${data.version}">
+  <Virksomhed>
+    <CVR>${data.cvr}</CVR>
+    <Navn>${company.name}</Navn>
+  </Virksomhed>
+  <Periode>
+    <Start>${data.periodeStart}</Start>
+    <Slut>${data.periodeSlut}</Slut>
+  </Periode>
+  <Angivelse>
+    <Salgsmoms>${data.angivelse.salgsmoms}</Salgsmoms>
+    <Koebsmoms>${data.angivelse.koebsmoms}</Koebsmoms>
+    <EUVarekoebMoms>${data.angivelse.euVarekoebMoms}</EUVarekoebMoms>
+    <YdelseskoebMoms>${data.angivelse.ydelseskoebMoms}</YdelseskoebMoms>
+    <SalgMedMoms>${data.angivelse.salgMedMoms}</SalgMedMoms>
+    <SalgUdenMoms>${data.angivelse.salgUdenMoms}</SalgUdenMoms>
+    <EUVarekoeb>${data.angivelse.euVarekoeb}</EUVarekoeb>
+    <Ydelseskoeb>${data.angivelse.ydelseskoeb}</Ydelseskoeb>
+  </Angivelse>
+  <Resultat>
+    <UdgaaendeMoms>${calculation.summary.totalOutputVAT}</UdgaaendeMoms>
+    <IndgaaendeMoms>${calculation.summary.totalInputVAT}</IndgaaendeMoms>
+    <NettoMoms>${calculation.summary.netVAT}</NettoMoms>
+  </Resultat>
+</Momsangivelse>`;
+}
+
+// =====================================================
+// HELPER FUNCTIONS
+// =====================================================
+
+/**
+ * Round to Danish ore (2 decimal places)
+ */
+function roundToOre(amount: number): number {
+  return Math.round(amount * 100) / 100;
+}
+
+/**
+ * Format period for SKAT export
+ */
+function formatPeriodForSKATExport(period: VATReportPeriod): string {
+  switch (period.periodicitet) {
+    case 'monthly':
+      return dayjs(period.startDate).format('YYYYMM');
+    case 'quarterly':
+      return `${period.year}Q${period.periodNumber}`;
+    case 'half-yearly':
+      return `${period.year}H${period.periodNumber}`;
+    case 'yearly':
+      return `${period.year}`;
+  }
+}
+
+/**
+ * Format VAT period for display
+ */
+export function formatVATPeriodDisplay(period: VATReportPeriod): string {
+  const MONTHS = [
+    'Januar', 'Februar', 'Marts', 'April', 'Maj', 'Juni',
+    'Juli', 'August', 'September', 'Oktober', 'November', 'December'
+  ];
+
+  switch (period.periodicitet) {
+    case 'monthly':
+      return `${MONTHS[period.periodNumber - 1]} ${period.year}`;
+    case 'quarterly':
+      return `Q${period.periodNumber} ${period.year}`;
+    case 'half-yearly':
+      return `H${period.periodNumber} ${period.year}`;
+    case 'yearly':
+      return `${period.year}`;
+  }
+}
+
+/**
+ * Create empty calculated VAT boxes
+ */
+export function createEmptyCalculatedBoxes(): VATReport['boxes'] {
+  const createBox = (id: VATBoxId | BasisBoxId): CalculatedVATBox => ({
+    ...SKAT_VAT_BOXES[id],
+    amount: 0,
+    transactionCount: 0,
+    transactionIds: [],
+  });
+
+  return {
+    A: createBox('A'),
+    B: createBox('B'),
+    C: createBox('C'),
+    D: createBox('D'),
+    '1': createBox('1'),
+    '2': createBox('2'),
+    '3': createBox('3'),
+    '4': createBox('4'),
+  };
+}
+
+/**
+ * Calculate days until VAT deadline
+ */
+export function getDaysUntilDeadline(deadline: string): number {
+  const today = dayjs();
+  const deadlineDate = dayjs(deadline);
+  return deadlineDate.diff(today, 'day');
+}
+
+/**
+ * Check if VAT deadline is approaching (within 14 days)
+ */
+export function isDeadlineApproaching(deadline: string): boolean {
+  const days = getDaysUntilDeadline(deadline);
+  return days >= 0 && days <= 14;
+}
+
+/**
+ * Check if VAT deadline is overdue
+ */
+export function isDeadlineOverdue(deadline: string): boolean {
+  return getDaysUntilDeadline(deadline) < 0;
+}
--- a/frontend/src/lib/vatCodes.ts
+++ b/frontend/src/lib/vatCodes.ts
@ -0,0 +1,378 @@
+// VAT Code Configuration for Danish SKAT Compliance
+
+import type {
+  VATCode,
+  VATCodeConfig,
+  VATPeriodicitetConfig,
+  SKATVATBox,
+  VATBoxId,
+  BasisBoxId,
+} from '@/types/vat';
+import type { VATPeriodicitet } from '@/types/periods';
+
+/**
+ * Complete VAT code configuration for Danish bookkeeping
+ */
+export const VAT_CODE_CONFIG: Record<VATCode, VATCodeConfig> = {
+  S25: {
+    code: 'S25',
+    nameDanish: 'Udgaaende moms 25%',
+    nameEnglish: 'Output VAT 25%',
+    rate: 0.25,
+    type: 'output',
+    affectsBoxes: {
+      vatBox: 'A',
+      basisBox: '1',
+    },
+    reverseCharge: false,
+    deductible: false,
+    description: 'Moms paa salg af varer og ydelser i Danmark',
+  },
+  K25: {
+    code: 'K25',
+    nameDanish: 'Indgaaende moms 25%',
+    nameEnglish: 'Input VAT 25%',
+    rate: 0.25,
+    type: 'input',
+    affectsBoxes: {
+      vatBox: 'B',
+    },
+    reverseCharge: false,
+    deductible: true,
+    description: 'Fradragsberettiget moms paa koeb',
+  },
+  EU_VARE: {
+    code: 'EU_VARE',
+    nameDanish: 'EU-varekoeb (erhvervelsesmoms)',
+    nameEnglish: 'EU goods purchase (acquisition VAT)',
+    rate: 0.25,
+    type: 'reverse_charge',
+    affectsBoxes: {
+      vatBox: 'C',
+      basisBox: '3',
+    },
+    reverseCharge: true,
+    deductible: true, // Both output and input VAT
+    description: 'Koeb af varer fra andre EU-lande med omvendt betalingspligt',
+  },
+  EU_YDELSE: {
+    code: 'EU_YDELSE',
+    nameDanish: 'EU-ydelseskoeb (omvendt betalingspligt)',
+    nameEnglish: 'EU services purchase (reverse charge)',
+    rate: 0.25,
+    type: 'reverse_charge',
+    affectsBoxes: {
+      vatBox: 'D',
+      basisBox: '4',
+    },
+    reverseCharge: true,
+    deductible: true,
+    description: 'Koeb af ydelser fra udlandet med omvendt betalingspligt',
+  },
+  MOMSFRI: {
+    code: 'MOMSFRI',
+    nameDanish: 'Momsfritaget',
+    nameEnglish: 'VAT exempt',
+    rate: 0,
+    type: 'exempt',
+    affectsBoxes: {
+      basisBox: '2',
+    },
+    reverseCharge: false,
+    deductible: false,
+    description: 'Momsfritaget salg (sundhed, undervisning, mv.)',
+  },
+  EKSPORT: {
+    code: 'EKSPORT',
+    nameDanish: 'Eksport (0%)',
+    nameEnglish: 'Export (0%)',
+    rate: 0,
+    type: 'exempt',
+    affectsBoxes: {
+      basisBox: '2',
+    },
+    reverseCharge: false,
+    deductible: false,
+    description: 'Eksport til lande uden for EU',
+  },
+  NONE: {
+    code: 'NONE',
+    nameDanish: 'Ingen moms',
+    nameEnglish: 'No VAT',
+    rate: 0,
+    type: 'none',
+    affectsBoxes: {},
+    reverseCharge: false,
+    deductible: false,
+    description: 'Transaktioner uden momsrelevans',
+  },
+};
+
+/**
+ * VAT period configuration based on SKAT requirements
+ */
+export const VAT_PERIODICITET_CONFIG: Record<VATPeriodicitet, VATPeriodicitetConfig> = {
+  monthly: {
+    type: 'monthly',
+    nameDanish: 'Maanedlig',
+    nameEnglish: 'Monthly',
+    deadlineDaysAfterPeriod: 25, // 25th of following month
+    periodsPerYear: 12,
+    threshold: { min: 50000000 }, // >50M DKK/year
+  },
+  quarterly: {
+    type: 'quarterly',
+    nameDanish: 'Kvartalsvis',
+    nameEnglish: 'Quarterly',
+    deadlineDaysAfterPeriod: 40, // ~40 days after quarter end (1st of 2nd month)
+    periodsPerYear: 4,
+    threshold: { min: 1000000, max: 50000000 }, // 1M-50M DKK/year
+  },
+  'half-yearly': {
+    type: 'half-yearly',
+    nameDanish: 'Halvaarslig',
+    nameEnglish: 'Half-yearly',
+    deadlineDaysAfterPeriod: 60, // ~2 months after period
+    periodsPerYear: 2,
+    threshold: { min: 300000, max: 1000000 }, // 300K-1M DKK/year
+  },
+  yearly: {
+    type: 'yearly',
+    nameDanish: 'Aarslig',
+    nameEnglish: 'Yearly',
+    deadlineDaysAfterPeriod: 90, // March 1st for calendar year
+    periodsPerYear: 1,
+    threshold: { max: 300000 }, // <300K DKK/year
+  },
+};
+
+/**
+ * SKAT VAT box definitions
+ */
+export const SKAT_VAT_BOXES: Record<VATBoxId | BasisBoxId, SKATVATBox> = {
+  // VAT amounts (Momsbeloeb)
+  A: {
+    id: 'A',
+    type: 'vat',
+    nameDanish: 'Salgsmoms',
+    nameEnglish: 'Output VAT (sales)',
+    description: 'Moms af varer og ydelser solgt i Danmark (25%)',
+    skippable: false,
+    isDeductible: false,
+  },
+  B: {
+    id: 'B',
+    type: 'vat',
+    nameDanish: 'Koebsmoms',
+    nameEnglish: 'Input VAT (purchases)',
+    description: 'Fradragsberettiget moms af koeb',
+    skippable: false,
+    isDeductible: true,
+  },
+  C: {
+    id: 'C',
+    type: 'vat',
+    nameDanish: 'Moms af EU-varekoeb',
+    nameEnglish: 'VAT on EU goods purchases',
+    description: 'Erhvervelsesmoms ved koeb af varer fra andre EU-lande',
+    skippable: true,
+    isDeductible: false, // Listed as output, but can be deducted via B
+  },
+  D: {
+    id: 'D',
+    type: 'vat',
+    nameDanish: 'Moms af ydelseskoeb fra udland',
+    nameEnglish: 'VAT on foreign services',
+    description: 'Moms ved koeb af ydelser fra udlandet med omvendt betalingspligt',
+    skippable: true,
+    isDeductible: false,
+  },
+  // Basis/turnover amounts (Omsaetning)
+  '1': {
+    id: '1',
+    type: 'basis',
+    nameDanish: 'Salg med moms',
+    nameEnglish: 'Sales with VAT',
+    description: 'Vaerdi af varer og ydelser solgt med dansk moms (momsgrundlag)',
+    skippable: false,
+    isDeductible: false,
+  },
+  '2': {
+    id: '2',
+    type: 'basis',
+    nameDanish: 'Salg uden moms',
+    nameEnglish: 'Sales without VAT',
+    description: 'Momsfrit salg og eksport',
+    skippable: true,
+    isDeductible: false,
+  },
+  '3': {
+    id: '3',
+    type: 'basis',
+    nameDanish: 'EU-varekoeb',
+    nameEnglish: 'EU goods purchases',
+    description: 'Vaerdi af varer koebt fra andre EU-lande',
+    skippable: true,
+    isDeductible: false,
+  },
+  '4': {
+    id: '4',
+    type: 'basis',
+    nameDanish: 'Ydelseskoeb fra udland',
+    nameEnglish: 'Foreign services purchases',
+    description: 'Vaerdi af ydelser koebt fra udlandet',
+    skippable: true,
+    isDeductible: false,
+  },
+};
+
+/**
+ * Default VAT accounts for automatic double-entry
+ */
+export const VAT_ACCOUNTS = {
+  inputVAT: '5610',    // Indgaaende moms (fradrag)
+  outputVAT: '5710',   // Udgaaende moms (skyld)
+  euVAT: '5620',       // EU-moms (erhvervelsesmoms)
+} as const;
+
+// =====================================================
+// HELPER FUNCTIONS
+// =====================================================
+
+/**
+ * Valid VAT code values for validation
+ */
+export const VALID_VAT_CODES: readonly VATCode[] = [
+  'S25', 'K25', 'EU_VARE', 'EU_YDELSE', 'MOMSFRI', 'EKSPORT', 'NONE'
+] as const;
+
+/**
+ * Check if a value is a valid VAT code
+ */
+export function isValidVATCode(code: unknown): code is VATCode {
+  return typeof code === 'string' && VALID_VAT_CODES.includes(code as VATCode);
+}
+
+/**
+ * Safely convert a string to VATCode, returns 'NONE' if invalid
+ */
+export function toVATCode(code: unknown): VATCode {
+  return isValidVATCode(code) ? code : 'NONE';
+}
+
+/**
+ * Get VAT code configuration
+ */
+export function getVATCodeConfig(code: VATCode): VATCodeConfig {
+  return VAT_CODE_CONFIG[code];
+}
+
+/**
+ * Get all VAT codes for a dropdown
+ */
+export function getVATCodeOptions(): Array<{ value: VATCode; label: string; description: string }> {
+  return Object.values(VAT_CODE_CONFIG).map((config) => ({
+    value: config.code,
+    label: `${config.code} - ${config.nameDanish}`,
+    description: config.description,
+  }));
+}
+
+/**
+ * Get VAT codes for expense transactions (input VAT)
+ */
+export function getExpenseVATCodeOptions(): Array<{ value: VATCode; label: string }> {
+  return Object.values(VAT_CODE_CONFIG)
+    .filter((config) => config.type === 'input' || config.type === 'reverse_charge' || config.type === 'none')
+    .map((config) => ({
+      value: config.code,
+      label: `${config.code} - ${config.nameDanish}`,
+    }));
+}
+
+/**
+ * Get VAT codes for income transactions (output VAT)
+ */
+export function getIncomeVATCodeOptions(): Array<{ value: VATCode; label: string }> {
+  return Object.values(VAT_CODE_CONFIG)
+    .filter((config) => config.type === 'output' || config.type === 'exempt' || config.type === 'none')
+    .map((config) => ({
+      value: config.code,
+      label: `${config.code} - ${config.nameDanish}`,
+    }));
+}
+
+/**
+ * Get period options for a dropdown
+ */
+export function getPeriodicitetOptions(): Array<{ value: VATPeriodicitet; label: string; description: string }> {
+  return Object.values(VAT_PERIODICITET_CONFIG).map((config) => ({
+    value: config.type,
+    label: config.nameDanish,
+    description: config.threshold?.min
+      ? `Omsaetning over ${(config.threshold.min / 1000000).toFixed(0)}M DKK`
+      : config.threshold?.max
+        ? `Omsaetning under ${(config.threshold.max / 1000000).toFixed(1)}M DKK`
+        : 'Standard',
+  }));
+}
+
+/**
+ * Get SKAT box definition
+ */
+export function getSKATBox(boxId: VATBoxId | BasisBoxId): SKATVATBox {
+  return SKAT_VAT_BOXES[boxId];
+}
+
+/**
+ * Check if a VAT code is deductible (affects input VAT)
+ */
+export function isVATDeductible(code: VATCode): boolean {
+  return VAT_CODE_CONFIG[code].deductible;
+}
+
+/**
+ * Check if a VAT code is reverse charge
+ */
+export function isReverseCharge(code: VATCode): boolean {
+  return VAT_CODE_CONFIG[code].reverseCharge;
+}
+
+/**
+ * Get VAT rate for a code
+ */
+export function getVATRate(code: VATCode): number {
+  return VAT_CODE_CONFIG[code].rate;
+}
+
+/**
+ * Calculate VAT from gross amount
+ * Note: Rounds net first, then calculates VAT as gross - net
+ * This ensures net + vat = gross (no floating point drift)
+ */
+export function calculateVATFromGross(grossAmount: number, rate: number): { net: number; vat: number } {
+  if (rate === 0) {
+    return { net: grossAmount, vat: 0 };
+  }
+  const rawNet = grossAmount / (1 + rate);
+  const net = Math.round(rawNet * 100) / 100;
+  // Calculate VAT as difference to guarantee gross = net + vat
+  const vat = Math.round((grossAmount - net) * 100) / 100;
+  return { net, vat };
+}
+
+/**
+ * Calculate VAT from net amount
+ * Note: Rounds VAT first, then calculates gross as net + vat
+ * This ensures net + vat = gross (no floating point drift)
+ */
+export function calculateVATFromNet(netAmount: number, rate: number): { gross: number; vat: number } {
+  if (rate === 0) {
+    return { gross: netAmount, vat: 0 };
+  }
+  const rawVat = netAmount * rate;
+  const vat = Math.round(rawVat * 100) / 100;
+  // Calculate gross as sum to guarantee net + vat = gross
+  const gross = Math.round((netAmount + vat) * 100) / 100;
+  return { gross, vat };
+}
--- a/frontend/src/main.tsx
+++ b/frontend/src/main.tsx
@ -0,0 +1,25 @@
+import React from 'react';
+import ReactDOM from 'react-dom/client';
+import { QueryClientProvider } from '@tanstack/react-query';
+import { ConfigProvider } from 'antd';
+import daDK from 'antd/locale/da_DK';
+import dayjs from 'dayjs';
+import 'dayjs/locale/da';
+
+import App from './App';
+import { queryClient } from './api/client';
+import { theme } from './styles/theme';
+import './styles/global.css';
+
+// Set dayjs locale globally
+dayjs.locale('da');
+
+ReactDOM.createRoot(document.getElementById('root')!).render(
+  <React.StrictMode>
+    <QueryClientProvider client={queryClient}>
+      <ConfigProvider theme={theme} locale={daDK}>
+        <App />
+      </ConfigProvider>
+    </QueryClientProvider>
+  </React.StrictMode>
+);
--- a/Show more
+++ b/Show more