feat(accounting): move Treuhänder export onto the Tax page

The standalone 'Treuhänder export' tab duplicated the Tax page's period/
currency filters over the same data. Fold the collective-journal export into
the Tax page as a third export action (target-tool format picker: generic /
Banana / bexio), beside Export CSV/PDF, with a link to its Chart-of-accounts
config. Removes the Accounting sub-nav 'export' tab (old /export route now
redirects to the Tax page); keeps Chart of accounts as its own setup tab.
Deletes the now-orphaned LedgerExportPage.

Build + JSON parse green.

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, BookOpen, FileSpreadsheet } from 'lucide-react';
+import { Landmark, Calculator, Inbox, Wallet, BookOpen } from 'lucide-react';
 import type { LucideIcon } from 'lucide-react';
 import { useFeatureFlags, type FeatureKey } from '../../contexts/FeatureFlagsContext';
 
@@ -45,18 +45,13 @@ export const AccountingLayout: React.FC = () => {
     },
     {
       key: 'tax-report',
+      // The Treuhänder export now lives ON the Tax page (same period/currency
+      // filters, same data) instead of a separate sub-tab — see TaxReportPage.
       to: '/admin/accounting/tax-report',
       label: t('accounting.subnav.taxReport', 'Tax'),
       icon: Calculator,
       featureFlag: 'taxReport',
     },
-    {
-      key: 'export',
-      to: '/admin/accounting/export',
-      label: t('accounting.subnav.export', 'Treuhänder export'),
-      icon: FileSpreadsheet,
-      featureFlag: 'taxReport',
-    },
     {
       key: 'ledger',
       to: '/admin/accounting/ledger',