feat(accounting): Accounting nav section + relocate Tax report out of CRM

Adds the `accounting` feature flag to the frontend (type, context default) and
a Settings -> Features toggle card. When enabled:

- A new top-level "Accounting" sidebar entry appears (gated by `accounting` +
  accounting.view), with an AccountingLayout sub-nav mirroring ClientsLayout.
- The Tax report relocates: it is HIDDEN from the CRM (Clients) sub-nav and
  shown under Accounting instead, at /admin/accounting/tax-report. When
  accounting is OFF, Tax stays under CRM exactly as before.

Tax visibility still depends on `taxReport` (which depends on `bills`), so the
relocation only changes WHERE the menu item lives, not whether it exists.

Files: featureFlags.service.ts (+'accounting'), FeatureFlagsContext default,
AdminSidebar entry, new AccountingLayout, ClientsLayout filter, App.tsx route,
FeaturesTab card, en/de i18n (navigation.accounting, accounting.*,
settings.features.accounting; DE authored natively).

Verified: `npm run build` green; en/de JSON valid.

frontend/src/App.tsx

@@ -64,6 +64,7 @@ import {
 import { CustomerAuthProvider } from './contexts/CustomerAuthContext';
 import { AdminLayout, AdminAuthWrapper } from './components/admin';
 import { ClientsLayout } from './components/admin/ClientsLayout';
+import { AccountingLayout } from './components/admin/AccountingLayout';
 import { RequireFeature } from './components/admin/RequireFeature';
 import { PageErrorBoundary, OfflineIndicator, SkipLink, DynamicFavicon, RobotsMetaTags, CMSContentBlock, Loading } from './components/common';
 import { MaintenanceWrapper } from './components/MaintenanceWrapper';
@@ -260,6 +261,20 @@ function App() {
                         </Route>
                       </Route>
 
+                      {/* Accounting section (migration 122). Parent gated by
+                          the `accounting` flag. Hosts the Tax report — which
+                          relocates here from the CRM sub-nav when accounting
+                          is on — plus the future inbound-invoice / expenses
+                          pages. Each sub-route is independently flagged. */}
+                      <Route element={<RequireFeature flag="accounting" />}>
+                        <Route path="accounting" element={<AccountingLayout />}>
+                          <Route element={<RequireFeature flag="taxReport" />}>
+                            <Route path="tax-report" element={<TaxReportPage />} />
+                          </Route>
+                          <Route index element={<Navigate to="/admin/accounting/tax-report" replace />} />
+                        </Route>
+                      </Route>
+
                       {/* Old /admin/customers paths now live under
                           /admin/clients/accounts. Kept indefinitely as
                           redirects so existing bookmarks and email links