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/migrations/core/133_accounting_on_when_bills.js

@@ -0,0 +1,34 @@
+/**
+ * Migration 133: invoices (Bills) force-enable the Accounting master.
+ *
+ * Invoice VAT config (codes + label) and the default hourly rate now live under
+ * Settings → Accounting, so an install with Bills enabled must have Accounting
+ * available. `applyDependencyRules` enforces this on every flag READ/WRITE, but
+ * the `requireFeatureFlag('accounting')` middleware reads the STORED row
+ * directly — so existing installs that already have `bills=true, accounting=false`
+ * would show the Accounting tab yet 403 its endpoints. This one-time correction
+ * brings the stored value in line (forward fix, not a compensation: it encodes a
+ * new dependency rule, it doesn't patch a buggy earlier migration).
+ *
+ * Idempotent: only flips accounting ON where Bills is on; never turns it off.
+ */
+function isOn(row) {
+  return !!(row && (row.value === true || row.value === 1 || row.value === '1'));
+}
+
+exports.up = async function (knex) {
+  if (!(await knex.schema.hasTable('feature_flags'))) return;
+  const bills = await knex('feature_flags').where({ key: 'bills' }).first();
+  if (!isOn(bills)) return;
+
+  const accounting = await knex('feature_flags').where({ key: 'accounting' }).first();
+  if (!accounting) {
+    await knex('feature_flags').insert({ key: 'accounting', value: true });
+  } else if (!isOn(accounting)) {
+    await knex('feature_flags').where({ key: 'accounting' }).update({ value: true });
+  }
+};
+
+// No down — we can't know whether Accounting was independently wanted, and
+// turning it back off could hide a section the admin now relies on.
+exports.down = async function () {};