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/App.tsx

@@ -70,7 +70,6 @@ import { AccountingLayout, AccountingIndex } from './components/admin/Accounting
 import { AccountingInboxPage } from './pages/admin/accounting/AccountingInboxPage';
 import { ExpensesLedgerPage } from './pages/admin/accounting/ExpensesLedgerPage';
 import { ChartOfAccountsPage } from './pages/admin/accounting/ChartOfAccountsPage';
-import { LedgerExportPage } from './pages/admin/accounting/LedgerExportPage';
 import { RequireFeature } from './components/admin/RequireFeature';
 import { PageErrorBoundary, OfflineIndicator, SkipLink, DynamicFavicon, RobotsMetaTags, CMSContentBlock, Loading } from './components/common';
 import { MaintenanceWrapper } from './components/MaintenanceWrapper';
@@ -288,7 +287,9 @@ function App() {
                           </Route>
                           <Route element={<RequireFeature flag="taxReport" />}>
                             <Route path="tax-report" element={<TaxReportPage />} />
-                            <Route path="export" element={<LedgerExportPage />} />
+                            {/* Treuhänder export moved onto the Tax page; keep
+                                the old path working for bookmarks. */}
+                            <Route path="export" element={<Navigate to="/admin/accounting/tax-report" replace />} />
                           </Route>
                           {/* Chart of accounts + VAT codes (Layer A) — gated by
                               the accounting master flag alongside the section. */}