feat(accounting): Accounting nav section + relocate Tax report out of CRM

Adds the `accounting` feature flag to the frontend (type, context default) and
a Settings -> Features toggle card. When enabled:

- A new top-level "Accounting" sidebar entry appears (gated by `accounting` +
  accounting.view), with an AccountingLayout sub-nav mirroring ClientsLayout.
- The Tax report relocates: it is HIDDEN from the CRM (Clients) sub-nav and
  shown under Accounting instead, at /admin/accounting/tax-report. When
  accounting is OFF, Tax stays under CRM exactly as before.

Tax visibility still depends on `taxReport` (which depends on `bills`), so the
relocation only changes WHERE the menu item lives, not whether it exists.

Files: featureFlags.service.ts (+'accounting'), FeatureFlagsContext default,
AdminSidebar entry, new AccountingLayout, ClientsLayout filter, App.tsx route,
FeaturesTab card, en/de i18n (navigation.accounting, accounting.*,
settings.features.accounting; DE authored natively).

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

frontend/src/i18n/locales/de.json

@@ -196,6 +196,7 @@
     "users": "Benutzer",
     "calendar": "Kalender",
     "clients": "CRM",
+    "accounting": "Buchhaltung",
     "betaTag": "Beta"
   },
   "eventTypes": {
@@ -1635,6 +1636,11 @@
         "sidebar": "Steuer",
         "requiresBills": "Bitte zuerst Rechnungen aktivieren — die Steuerliste liest aus Ihren Rechnungen."
       },
+      "accounting": {
+        "title": "Buchhaltung",
+        "description": "Eingehende Lieferantenrechnungen erfassen (Upload oder Handy-/Tablet-Kamera), Aufwände kategorisieren und Kosten dem passenden Event des Kunden weiterverrechnen. Wenn aktiviert, wandert die Steuerliste aus dem CRM in einen eigenen Buchhaltungsbereich. MwSt-/Steuerbehandlung dient nur als Orientierung — vor dem Verlassen darauf mit Ihrem Treuhänder prüfen.",
+        "sidebar": "Buchhaltung"
+      },
       "analytics": {
         "title": "Statistiken",
         "description": "Speichernutzung, Galerie-Aufrufe, Download-Zahlen und Statistiken pro Veranstaltung."
@@ -3375,6 +3381,18 @@
       "development": "Entwicklung"
     }
   },
+  "accounting": {
+    "title": "Buchhaltung",
+    "subtitle": "Eingehende Lieferantenrechnungen, Aufwände und Auswertungen.",
+    "navAriaLabel": "Buchhaltungs-Navigation",
+    "empty": {
+      "title": "Keine Buchhaltungsfunktionen aktiviert",
+      "body": "Aktiviere die Steuerliste (oder eine andere Buchhaltungs-Unterfunktion) unter Einstellungen → Funktionen, um loszulegen."
+    },
+    "subnav": {
+      "taxReport": "Steuer"
+    }
+  },
   "calendar": {
     "pageTitle": "Kalender",
     "subtitle": "Termine, erfasste Stunden und offene Angebote/Verträge in einer Ansicht.",

frontend/src/i18n/locales/en.json

@@ -196,6 +196,7 @@
     "users": "Users",
     "calendar": "Calendar",
     "clients": "CRM",
+    "accounting": "Accounting",
     "betaTag": "Beta"
   },
   "archives": {
@@ -1193,6 +1194,11 @@
         "sidebar": "Tax",
         "requiresBills": "Enable Bills first — the tax report reads from your invoices."
       },
+      "accounting": {
+        "title": "Accounting",
+        "description": "Capture incoming supplier invoices (upload or phone/tablet camera), categorize expenses and re-bill costs to clients on the relevant event. When enabled, the Tax report moves out of CRM into a dedicated Accounting section. VAT / tax treatment is provided as guidance only — verify with your Treuhänder before relying on it.",
+        "sidebar": "Accounting"
+      },
       "analytics": {
         "title": "Analytics",
         "description": "Storage usage, gallery views, download counts, and per-event stats."
@@ -3375,6 +3381,18 @@
       "development": "Development"
     }
   },
+  "accounting": {
+    "title": "Accounting",
+    "subtitle": "Inbound supplier invoices, expenses and reporting.",
+    "navAriaLabel": "Accounting navigation",
+    "empty": {
+      "title": "No accounting features enabled",
+      "body": "Enable the Tax report (or another accounting sub-feature) under Settings → Features to get started."
+    },
+    "subnav": {
+      "taxReport": "Tax"
+    }
+  },
   "calendar": {
     "pageTitle": "Calendar",
     "subtitle": "Events, logged hours, and pending quotes/contracts in one view.",