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/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 } from 'lucide-react';
+import { Landmark, Calculator, Inbox, Wallet } from 'lucide-react';
 import type { LucideIcon } from 'lucide-react';
 import { useFeatureFlags, type FeatureKey } from '../../contexts/FeatureFlagsContext';
 
@@ -36,6 +36,13 @@ export const AccountingLayout: React.FC = () => {
       icon: Inbox,
       featureFlag: 'incomingInvoices',
     },
+    {
+      key: 'expenses',
+      to: '/admin/accounting/expenses',
+      label: t('accounting.subnav.expenses', 'Expenses'),
+      icon: Wallet,
+      featureFlag: 'incomingInvoices',
+    },
     {
       key: 'tax-report',
       to: '/admin/accounting/tax-report',