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.

backend/src/routes/adminExpenses.js

@@ -43,17 +43,20 @@ const inboundUpload = multer({
   },
 });
 
-async function requireAccountingFlag(req, res, next) {
+// Gated on the `incomingInvoices` sub-feature, which the feature-flag
+// dependency rules force OFF whenever the `accounting` master is off — so
+// this single check covers both.
+async function requireIncomingInvoicesFlag(req, res, next) {
   try {
-    const row = await db('feature_flags').where({ key: 'accounting' }).first();
+    const row = await db('feature_flags').where({ key: 'incomingInvoices' }).first();
     const enabled = row && (row.value === true || row.value === 1 || row.value === '1');
-    if (!enabled) return res.status(403).json({ error: 'Accounting feature is disabled', code: 'ACCOUNTING_DISABLED' });
+    if (!enabled) return res.status(403).json({ error: 'Incoming invoices feature is disabled', code: 'INCOMING_INVOICES_DISABLED' });
     return next();
   } catch (err) { return next(err); }
 }
 
 router.use(adminAuth);
-router.use(requireAccountingFlag);
+router.use(requireIncomingInvoicesFlag);
 
 // ── Expense categories (literal path — register BEFORE '/:id') ──────────────
 router.get('/categories', requirePermission('accounting.view'), handleAsync(async (_req, res) => {

backend/src/routes/adminFeatureFlags.js

@@ -66,6 +66,10 @@ const KNOWN_FLAGS = [
   // supplier invoices, expenses + re-bill, and the tax report (which
   // relocates here from CRM when this is on). Strictly opt-in.
   'accounting',
+  // Incoming invoices (migration 124) — supplier-invoice capture +
+  // expenses + re-bill. Accounting sub-feature; forced off when the
+  // `accounting` master is off.
+  'incomingInvoices',
 ];
 
 // Spec defaults for any flag missing from the DB (e.g. a row added by a
@@ -89,6 +93,7 @@ const DEFAULT_FLAGS = {
   hoursLogging: false,
   contracts: false,
   accounting: false,
+  incomingInvoices: false,
 };
 
 async function readAllFlags() {
@@ -109,10 +114,13 @@ function applyDependencyRules(flags) {
   // Sub-features can't outlive their parents.
   if (out.quotes === false) out.bills = false;
   if (out.calendar === false) out.calendarBooking = false;
-  // Tax report only makes sense when bills are on — turning bills off
-  // implicitly turns the tax report off too. Admins enabling tax
-  // report must first enable bills.
-  if (out.bills === false) out.taxReport = false;
+  // 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).
+  if (out.accounting === false) {
+    out.taxReport = false;
+    out.incomingInvoices = false;
+  }
   // Clients parent flag is DERIVED from its children. Admins don't
   // toggle it directly in the Features tab — they enable a specific
   // sub-feature (Accounts today; Calendar/Quotes/Bills/Messaging
@@ -125,9 +133,10 @@ function applyDependencyRules(flags) {
     || out.crmDevelopment
     || out.quotes
     || out.bills
-    || out.taxReport
     || out.hoursLogging
     || out.contracts
+    // NOTE: taxReport intentionally removed — Tax export moved to the
+    // Accounting section (its own master), no longer a CRM sub-feature.
     // Migration 137 — admin calendar lights up the Clients section.
     // (calendarBooking is gated behind `calendar` so adding the parent
     // is sufficient.)