refactor(accounting): make Accounting a master flag with sub-toggles

Replaces the earlier peer-`accounting` flag (which only *conditionally*
relocated Tax) with a cleaner top-level master + sub-toggle model, per design
discussion:

- `accounting` = explicit top-level MASTER (Settings -> Features). Off hides
  the whole Accounting section.
- Sub-toggles, gated under the master:
  - `taxReport` ("Tax export") moves PERMANENTLY out of CRM. Removed from the
    Clients sub-nav and from the derived `clients` flag. Now INDEPENDENT of
    Bills (per decision). Old /admin/clients/tax-report -> redirect to
    /admin/accounting/tax-report.
  - `incomingInvoices` (new) gates the supplier-invoice capture / expenses /
    re-bill feature; the /api/admin/expenses router now checks it.
- Dependency rules (backend + frontend): accounting off forces taxReport +
  incomingInvoices off; taxReport dropped from the clients derivation; the
  bills->taxReport rule removed.
- Preserve visuals: migration 122 rewritten to auto-enable `accounting` on
  installs that already had Tax on (so the tab doesn't vanish), and to seed
  `incomingInvoices` off. Verified with a SQLite harness (taxReport on ->
  accounting on; off -> off).
- Settings -> Features: new "Accounting" section with the master card + Tax
  export + Incoming invoices sub-cards (disabled until the master is on).
- i18n: navigation.accounting, accounting.*, settings.features.{accounting,
  incomingInvoices,taxReport.requiresAccounting}, sections.accounting (EN + DE,
  DE authored natively); Tax report relabelled "Tax export"/"Steuerexport".

Verified: node -c, migration-122 harness, en/de JSON valid, npm run build green.

frontend/src/i18n/locales/de.json

@@ -1585,6 +1585,7 @@
         "communication": "Kommunikation",
         "scheduling": "Terminplanung",
         "sales": "Vertrieb",
+        "accounting": "Buchhaltung",
         "insights": "Auswertungen & Zugriff",
         "customers": "Kunden",
         "clients": "CRM"
@@ -1631,16 +1632,23 @@
         "sidebar": "Rechnungen"
       },
       "taxReport": {
-        "title": "Steuerliste",
+        "title": "Steuerexport",
         "description": "Periodenbezogene Umsatzliste mit Netto- und MwSt-Aufschlüsselung, gruppiert nach MwSt-Satz. Exportieren Sie als PDF (Querformat, Firmenbriefkopf) oder CSV für Ihre Buchhaltung. Stornierte Rechnungen bleiben für die lückenlose Nummernfolge sichtbar, sind aber nicht in den Summen enthalten.",
         "sidebar": "Steuer",
-        "requiresBills": "Bitte zuerst Rechnungen aktivieren — die Steuerliste liest aus Ihren Rechnungen."
+        "requiresBills": "Bitte zuerst Rechnungen aktivieren — die Steuerliste liest aus Ihren Rechnungen.",
+        "requiresAccounting": "Bitte zuerst Buchhaltung aktivieren — der Steuerexport liegt im Buchhaltungsbereich."
       },
       "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.",
+        "description": "Ein eigener Buchhaltungsbereich, getrennt vom CRM. Hier aktivieren und dann die Unterfunktionen unten einschalten (Steuerexport, Eingangsrechnungen). MwSt-/Steuerbehandlung dient nur als Orientierung — vor dem Verlassen darauf mit Ihrem Treuhänder prüfen.",
         "sidebar": "Buchhaltung"
       },
+      "incomingInvoices": {
+        "title": "Eingangsrechnungen",
+        "description": "Eingehende Lieferantenrechnungen erfassen (Upload oder Handy-/Tablet-Kamera), Aufwände kategorisieren und Kosten dem passenden Event des Kunden mit vertraglich hinterlegtem Zuschlag weiterverrechnen.",
+        "sidebar": "Eingang",
+        "requiresAccounting": "Bitte zuerst Buchhaltung aktivieren — Eingangsrechnungen liegen im Buchhaltungsbereich."
+      },
       "analytics": {
         "title": "Statistiken",
         "description": "Speichernutzung, Galerie-Aufrufe, Download-Zahlen und Statistiken pro Veranstaltung."

frontend/src/i18n/locales/en.json

@@ -1143,6 +1143,7 @@
         "communication": "Communication",
         "scheduling": "Scheduling",
         "sales": "Sales",
+        "accounting": "Accounting",
         "insights": "Insights & Access",
         "customers": "Customers",
         "clients": "CRM"
@@ -1189,16 +1190,23 @@
         "sidebar": "Invoices"
       },
       "taxReport": {
-        "title": "Tax report",
+        "title": "Tax export",
         "description": "Period-scoped revenue list with net + VAT breakdown grouped by VAT rate. Export as PDF (landscape, company letterhead) or CSV for your accountant. Cancelled invoices stay visible for a gap-free audit trail but are excluded from totals.",
         "sidebar": "Tax",
-        "requiresBills": "Enable Bills first — the tax report reads from your invoices."
+        "requiresBills": "Enable Bills first — the tax report reads from your invoices.",
+        "requiresAccounting": "Enable Accounting first — Tax export lives in the Accounting section."
       },
       "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.",
+        "description": "A dedicated Accounting area, separate from CRM. Turn this on, then enable the sub-features below (Tax export, Incoming invoices). VAT / tax treatment is guidance only — verify with your Treuhänder before relying on it.",
         "sidebar": "Accounting"
       },
+      "incomingInvoices": {
+        "title": "Incoming invoices",
+        "description": "Capture received supplier invoices (upload or phone/tablet camera), categorize expenses, and re-bill costs to clients on the relevant event with a contract-driven markup.",
+        "sidebar": "Incoming",
+        "requiresAccounting": "Enable Accounting first — Incoming invoices live in the Accounting section."
+      },
       "analytics": {
         "title": "Analytics",
         "description": "Storage usage, gallery views, download counts, and per-event stats."