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/i18n/locales/de.json

@@ -3400,6 +3400,7 @@
     },
     "subnav": {
       "incomingInvoices": "Eingangsrechnungen",
+      "expenses": "Aufwände",
       "taxReport": "Steuer"
     },
     "disposition": {
@@ -3449,6 +3450,36 @@
         "eventId": "Event-ID (optional)",
         "markup": "Zuschlag"
       }
+    },
+    "expenseStatus": {
+      "open": "Offen",
+      "parked": "Geparkt",
+      "billed": "Verrechnet",
+      "declined": "Abgelehnt"
+    },
+    "paymentMethod": {
+      "bank_transfer": "Überweisung",
+      "cash": "Bargeld",
+      "twint": "TWINT",
+      "paypal": "PayPal",
+      "card": "Karte",
+      "other": "Sonstiges"
+    },
+    "ledger": {
+      "allStatuses": "Alle Status",
+      "allDispositions": "Alle Zuordnungen",
+      "empty": "Noch keine Aufwände — Dokumente im Eingang kategorisieren.",
+      "untitled": "Aufwand",
+      "invoiceLink": "Rechnung",
+      "paid": "Bezahlt",
+      "markPaid": "Als bezahlt markieren",
+      "markPaidTitle": "Lieferant als bezahlt markieren",
+      "paidDate": "Zahldatum",
+      "method": "Methode",
+      "reference": "Referenz (optional)",
+      "confirmPaid": "Als bezahlt markieren",
+      "paidToast": "Als bezahlt markiert.",
+      "unpaidToast": "Als nicht bezahlt markiert."
     }
   },
   "calendar": {

frontend/src/i18n/locales/en.json

@@ -3400,6 +3400,7 @@
     },
     "subnav": {
       "incomingInvoices": "Incoming invoices",
+      "expenses": "Expenses",
       "taxReport": "Tax"
     },
     "disposition": {
@@ -3449,6 +3450,36 @@
         "eventId": "Event ID (optional)",
         "markup": "Markup"
       }
+    },
+    "expenseStatus": {
+      "open": "Open",
+      "parked": "Parked",
+      "billed": "Billed",
+      "declined": "Declined"
+    },
+    "paymentMethod": {
+      "bank_transfer": "Bank transfer",
+      "cash": "Cash",
+      "twint": "TWINT",
+      "paypal": "PayPal",
+      "card": "Card",
+      "other": "Other"
+    },
+    "ledger": {
+      "allStatuses": "All statuses",
+      "allDispositions": "All dispositions",
+      "empty": "No expenses yet — categorize documents in the inbox.",
+      "untitled": "Expense",
+      "invoiceLink": "Invoice",
+      "paid": "Paid",
+      "markPaid": "Mark paid",
+      "markPaidTitle": "Mark supplier paid",
+      "paidDate": "Payment date",
+      "method": "Method",
+      "reference": "Reference (optional)",
+      "confirmPaid": "Mark paid",
+      "paidToast": "Marked as paid.",
+      "unpaidToast": "Marked as not paid."
     }
   },
   "calendar": {