feat(accounting): Accounting settings tab (km / per-diem rate, require-proof)

New Settings -> Accounting tab (gated by the accounting flag) to edit the km
rate, per-diem rate and the "require proof for expense" toggle (reads GET /
writes PUT /admin/settings/accounting). Rates are CHF, stored as integer minor
units; carries the "verify with your Treuhaender" disclaimer. Wired into
SettingsPage (TabType, keys, flag-gated nav item, render) + the features barrel.

i18n: settings.accounting.* (EN + DE, DE native).
Verified: tsc --noEmit clean (0 errors); en/de JSON valid; npm run build green.

frontend/src/i18n/locales/de.json

@@ -1705,6 +1705,17 @@
     },
     "crm": {
       "title": "CRM-Verhalten"
+    },
+    "accounting": {
+      "title": "Buchhaltung",
+      "subtitle": "Standardsätze für interne Aufwände und die Belegpflicht.",
+      "kmRate": "Kilometersatz (CHF / km)",
+      "kmRateHint": "Standard für Kilometer-Aufwände; pro Eintrag überschreibbar.",
+      "perDiemRate": "Spesenpauschale (CHF / Tag)",
+      "perDiemRateHint": "Standard für Pauschal-Aufwände; pro Eintrag überschreibbar.",
+      "requireProof": "Beleg für jeden Aufwand verlangen",
+      "disclaimer": "Sätze und MwSt-/Steuerbehandlung dienen nur als Orientierung — mit Ihrem Treuhänder prüfen.",
+      "savedToast": "Buchhaltungseinstellungen gespeichert."
     }
   },
   "branding": {

frontend/src/i18n/locales/en.json

@@ -1263,6 +1263,17 @@
     },
     "crm": {
       "title": "CRM behaviour"
+    },
+    "accounting": {
+      "title": "Accounting",
+      "subtitle": "Default rates for internal expenses and the proof requirement.",
+      "kmRate": "Mileage rate (CHF / km)",
+      "kmRateHint": "Default applied to mileage expenses; overridable per entry.",
+      "perDiemRate": "Per-diem rate (CHF / day)",
+      "perDiemRateHint": "Default applied to per-diem expenses; overridable per entry.",
+      "requireProof": "Require a proof file on every expense",
+      "disclaimer": "Rates and VAT/tax treatment are guidance only — verify with your Treuhänder.",
+      "savedToast": "Accounting settings saved."
     }
   },
   "analytics": {