feat(accounting): expenses ledger + supplier-payment toggle

Adds Accounting → Expenses, the view of everything triaged out of the inbox:

- ExpensesLedgerPage: filter by status / disposition; each row shows the
  disposition + status badge, CHF amount, created date, and a link to the
  client invoice for re-billed items. Supplier-payment toggle ("Mark paid" ->
  method + date + reference modal; "Paid" -> click to revert) wired to
  /:id/supplier-payment. Payment status is decoupled from categorisation, per
  the locked design; declined/duplicate rows skip the toggle.
- AccountingLayout: "Expenses" sub-nav item (gated by incomingInvoices).
- App.tsx: /admin/accounting/expenses route.
- i18n: accounting.subnav.expenses, accounting.ledger/expenseStatus/
  paymentMethod (EN + DE, DE authored natively).

Verified: en/de JSON valid; npm run build green.

frontend/src/App.tsx

@@ -66,6 +66,7 @@ import { AdminLayout, AdminAuthWrapper } from './components/admin';
 import { ClientsLayout } from './components/admin/ClientsLayout';
 import { AccountingLayout, AccountingIndex } from './components/admin/AccountingLayout';
 import { AccountingInboxPage } from './pages/admin/accounting/AccountingInboxPage';
+import { ExpensesLedgerPage } from './pages/admin/accounting/ExpensesLedgerPage';
 import { RequireFeature } from './components/admin/RequireFeature';
 import { PageErrorBoundary, OfflineIndicator, SkipLink, DynamicFavicon, RobotsMetaTags, CMSContentBlock, Loading } from './components/common';
 import { MaintenanceWrapper } from './components/MaintenanceWrapper';
@@ -271,6 +272,7 @@ function App() {
                         <Route path="accounting" element={<AccountingLayout />}>
                           <Route element={<RequireFeature flag="incomingInvoices" />}>
                             <Route path="inbox" element={<AccountingInboxPage />} />
+                            <Route path="expenses" element={<ExpensesLedgerPage />} />
                           </Route>
                           <Route element={<RequireFeature flag="taxReport" />}>
                             <Route path="tax-report" element={<TaxReportPage />} />