feat(accounting): invoices force-enable the Accounting master

Invoice VAT config (codes + label) and the hourly rate now live under
Settings → Accounting, so an install with Invoices must have Accounting
available.

- applyDependencyRules (backend adminFeatureFlags.js + frontend
  FeatureFlagsContext.tsx): bills on → accounting on, before the
  accounting→children rule so the sub-features keep their own state.
- Migration 133 corrects existing installs: set the STORED accounting=true
  where bills is on. requireFeatureFlag('accounting') reads the raw row, so
  without this an upgraded install (invoices on, accounting off) would show
  the tab but 403 its endpoints. Idempotent; only flips on; no down.
- Features tab: the Accounting card shows locked-on (disabled + hint) while
  Invoices is enabled.

Also includes the i18n keys (en/de) for the VAT/financial settings move.

backend/src/routes/adminFeatureFlags.js

@@ -127,6 +127,11 @@ function applyDependencyRules(flags) {
   // Sub-features can't outlive their parents.
   if (out.quotes === false) out.bills = false;
   if (out.calendar === false) out.calendarBooking = false;
+  // Invoices (Bills) force-enable the Accounting master: invoice VAT config
+  // (codes + label) and the hourly rate live under Settings → Accounting, so
+  // an install with invoices must have Accounting available. Runs BEFORE the
+  // accounting→children rule so the sub-features keep their own stored state.
+  if (out.bills === true) out.accounting = true;
   // Accounting is a top-level MASTER; its sub-features can't outlive it.
   // Tax export is now independent of Bills — it relocated permanently
   // into the Accounting section (its own master gate).