Fix company and fiscal year sync issues

- Remove duplicate sync logic from CompanySwitcher (CompanyGuard handles it)
- Validate persisted activeCompany against API response in CompanyGuard
- Reset fiscal year state when switching companies in FiscalYearSelector
- Validate currentFiscalYear belongs to current company's data

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/auth/CompanyGuard.tsx

@@ -31,30 +31,20 @@ export default function CompanyGuard({ children }: CompanyGuardProps) {
 
   // Sync companies to store when loaded
   useEffect(() => {
-    if (companies) {
+    if (companies && companies.length > 0) {
       setCompanies(companies);
 
-      // Set active company if not already set
-      if (companies.length > 0 && !activeCompany) {
-        // Try to restore from localStorage
-        const stored = localStorage.getItem('books-company-storage');
-        if (stored) {
-          try {
-            const parsed = JSON.parse(stored);
-            const storedCompany = parsed.state?.activeCompany;
-            if (storedCompany) {
-              const found = companies.find((c) => c.id === storedCompany.id);
-              if (found) {
-                setActiveCompany(found);
-                return;
-              }
-            }
-          } catch {
-            // Ignore parse errors
-          }
-        }
-        // Default to first company
+      // Validate that activeCompany exists in API response
+      const validCompany = activeCompany
+        ? companies.find((c) => c.id === activeCompany.id)
+        : null;
+
+      if (!validCompany) {
+        // Persisted company doesn't exist - set to first
         setActiveCompany(companies[0]);
+      } else if (JSON.stringify(validCompany) !== JSON.stringify(activeCompany)) {
+        // Update with fresh data from API
+        setActiveCompany(validCompany);
       }
     }
   }, [companies, activeCompany, setCompanies, setActiveCompany]);

frontend/src/components/layout/CompanySwitcher.tsx

@@ -1,8 +1,6 @@
-import { useEffect } from 'react';
 import { Select, Space, Typography, Tag, Skeleton } from 'antd';
 import { ShopOutlined } from '@ant-design/icons';
 import { useCompanyStore } from '@/stores/companyStore';
-import { useMyCompanies } from '@/api/queries/companyQueries';
 import { formatCVR } from '@/lib/formatters';
 import type { Company } from '@/types/accounting';
 
@@ -13,18 +11,8 @@ interface CompanySwitcherProps {
 }
 
 export default function CompanySwitcher({ compact = false }: CompanySwitcherProps) {
-  const { data: companies = [], isLoading } = useMyCompanies();
-  const { activeCompany, setActiveCompany, setCompanies } = useCompanyStore();
-
-  // Sync companies with store when data changes
-  useEffect(() => {
-    if (companies.length > 0) {
-      setCompanies(companies);
-      if (!activeCompany) {
-        setActiveCompany(companies[0]);
-      }
-    }
-  }, [companies, activeCompany, setActiveCompany, setCompanies]);
+  const { activeCompany, setActiveCompany } = useCompanyStore();
+  const companies = useCompanyStore((state) => state.companies);
 
   const handleCompanyChange = (companyId: string) => {
     const company = companies.find((c) => c.id === companyId);
@@ -33,12 +21,8 @@ export default function CompanySwitcher({ compact = false }: CompanySwitcherProp
     }
   };
 
-  if (isLoading) {
-    return <Skeleton.Input style={{ width: 200 }} active />;
-  }
-
   if (companies.length === 0) {
-    return null;
+    return <Skeleton.Input style={{ width: 200 }} active />;
   }
 
   return (

frontend/src/components/layout/FiscalYearSelector.tsx

@@ -1,6 +1,6 @@
 // FiscalYearSelector - Dropdown for selecting active fiscal year (regnskabsar)
 
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useRef } from 'react';
 import { Select, Space, Typography, Tag, Divider, Button, Skeleton } from 'antd';
 import {
   CalendarOutlined,
@@ -62,21 +62,38 @@ export default function FiscalYearSelector({ onCreateNew, onManage }: FiscalYear
 
   const [createModalOpen, setCreateModalOpen] = useState(false);
 
-  // Sync fiscal years with store when data changes
+  // Track company changes
+  const prevCompanyIdRef = useRef<string | undefined>(activeCompany?.id);
+
+  // Reset fiscal year state when company changes
+  useEffect(() => {
+    if (activeCompany?.id !== prevCompanyIdRef.current) {
+      const isInitialMount = prevCompanyIdRef.current === undefined;
+      prevCompanyIdRef.current = activeCompany?.id;
+
+      if (!isInitialMount) {
+        // Company changed - reset fiscal year state
+        setFiscalYears([]);
+        setCurrentFiscalYear(null);
+      }
+    }
+  }, [activeCompany?.id, setFiscalYears, setCurrentFiscalYear]);
+
+  // Sync fiscal years from API to store
   useEffect(() => {
     if (fiscalYearsData.length > 0) {
       setFiscalYears(fiscalYearsData);
-    }
-  }, [fiscalYearsData, 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]);
+      // Validate currentFiscalYear belongs to this company's data
+      const isValid = currentFiscalYear &&
+        fiscalYearsData.some(fy => fy.id === currentFiscalYear.id);
+
+      if (!isValid) {
+        const openYear = fiscalYearsData.find(y => y.status === 'open');
+        setCurrentFiscalYear(openYear || fiscalYearsData[0]);
+      }
     }
-  }, [fiscalYears, currentFiscalYear, setCurrentFiscalYear]);
+  }, [fiscalYearsData, currentFiscalYear, setFiscalYears, setCurrentFiscalYear]);
 
   const handleFiscalYearChange = (yearId: string) => {
     const year = fiscalYears.find((y) => y.id === yearId);