feat(accounting): Layer A frontend — chart of accounts CRUD + Treuhänder export UI

- ledger.service.ts: accounts/VAT-codes/mappings CRUD + export client
- ChartOfAccountsPage: full CRUD for the Swiss/LI KMU chart + MWST codes,
  category→account mapping, default/system accounts + tax-treatment/rate→VAT
  maps, with a 'guideline only' note
- LedgerExportPage: period + currency + target tool (generic/Banana/bexio)
  → collective-journal CSV download, with accrual-only + Treuhänder disclaimer
- AccountingLayout: 'Treuhänder export' (taxReport flag) + 'Chart of accounts'
  (accounting flag) sub-nav entries; App routes wired
- en/de translations (accounting.taxTreatment.* enum + ledger.* namespace)

de/en authored natively; no machine-translated locales touched here.

frontend/src/components/admin/AccountingLayout.tsx

@@ -9,7 +9,7 @@
 import React from 'react';
 import { NavLink, Outlet, Navigate, useLocation, useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
-import { Landmark, Calculator, Inbox, Wallet } from 'lucide-react';
+import { Landmark, Calculator, Inbox, Wallet, BookOpen, FileSpreadsheet } from 'lucide-react';
 import type { LucideIcon } from 'lucide-react';
 import { useFeatureFlags, type FeatureKey } from '../../contexts/FeatureFlagsContext';
 
@@ -50,7 +50,21 @@ export const AccountingLayout: React.FC = () => {
       icon: Calculator,
       featureFlag: 'taxReport',
     },
-    // Future: expenses ledger, Erfolgsrechnung.
+    {
+      key: 'export',
+      to: '/admin/accounting/export',
+      label: t('accounting.subnav.export', 'Treuhänder export'),
+      icon: FileSpreadsheet,
+      featureFlag: 'taxReport',
+    },
+    {
+      key: 'ledger',
+      to: '/admin/accounting/ledger',
+      label: t('accounting.subnav.chartOfAccounts', 'Chart of accounts'),
+      icon: BookOpen,
+      featureFlag: 'accounting',
+    },
+    // Future: Erfolgsrechnung (Layer B).
   ];
 
   const enabledItems = navItems.filter((item) => flags[item.featureFlag]);