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/App.tsx

@@ -244,10 +244,10 @@ function App() {
                             />
                           </Route>
 
-                          {/* Tax / Steuer report — gated by `taxReport`. */}
-                          <Route element={<RequireFeature flag="taxReport" />}>
-                            <Route path="tax-report" element={<TaxReportPage />} />
-                          </Route>
+                          {/* Tax export moved permanently to the Accounting
+                              section. Keep this path as a redirect so old
+                              bookmarks / links don't 404. */}
+                          <Route path="tax-report" element={<Navigate to="/admin/accounting/tax-report" replace />} />
                           {/* Developer tools — gated by `crmDevelopment`. */}
                           <Route element={<RequireFeature flag="crmDevelopment" />}>
                             <Route path="development" element={<CrmDevelopmentPage />} />