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/components/admin/ClientsLayout.tsx

@@ -14,7 +14,7 @@
 import React from 'react';
 import { NavLink, Outlet, useLocation, useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
-import { Briefcase, UserCog, FileText, Receipt, Wrench, Calculator, Clock, ScrollText, Calendar } from 'lucide-react';
+import { Briefcase, UserCog, FileText, Receipt, Wrench, Clock, ScrollText, Calendar } from 'lucide-react';
 import type { LucideIcon } from 'lucide-react';
 import { useFeatureFlags, type FeatureKey } from '../../contexts/FeatureFlagsContext';
 
@@ -81,13 +81,8 @@ export const ClientsLayout: React.FC = () => {
       icon: Receipt,
       featureFlag: 'bills',
     },
-    {
-      key: 'tax-report',
-      to: '/admin/clients/tax-report',
-      label: t('clients.subnav.taxReport', 'Tax'),
-      icon: Calculator,
-      featureFlag: 'taxReport',
-    },
+    // Tax export moved permanently to the Accounting section (it is no
+    // longer a CRM sub-feature). See AccountingLayout.
     // Future sub-features:
     //   { key: 'messaging', ... featureFlag: 'messaging' }
     {
@@ -99,13 +94,7 @@ export const ClientsLayout: React.FC = () => {
     },
   ];
 
-  const enabledItems = navItems.filter((item) => {
-    if (!flags[item.featureFlag]) return false;
-    // When the Accounting area is enabled, the tax report relocates out
-    // of CRM and under Accounting — hide it here so it isn't in both.
-    if (item.key === 'tax-report' && flags.accounting) return false;
-    return true;
-  });
+  const enabledItems = navItems.filter((item) => flags[item.featureFlag]);
 
   // When the parent `clients` flag is on but no sub-feature is enabled,
   // there's nothing to render. Settings → Features is one click away