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/features/settings/tabs/FeaturesTab.tsx

@@ -16,6 +16,7 @@ import {
   Briefcase,
   Wrench,
   Calculator,
+  Landmark,
 } from 'lucide-react';
 import { useTranslation } from 'react-i18next';
 import { Button, Card } from '../../../components/common';
@@ -278,6 +279,20 @@ export const FeaturesTab: React.FC = () => {
             ) : undefined}
           />
 
+          <FeatureCard
+            icon={Landmark}
+            title={t('settings.features.accounting.title', 'Accounting')}
+            description={t(
+              'settings.features.accounting.description',
+              'Capture incoming supplier invoices (upload or phone/tablet camera), categorize expenses and re-bill costs to clients on the relevant event. When enabled, the Tax report moves out of CRM into a dedicated Accounting section. VAT / tax treatment is provided as guidance only — verify with your Treuhänder before relying on it.',
+            )}
+            status="new"
+            statusLabel={statusLabel('new')}
+            sidebarLabel={t('settings.features.accounting.sidebar', 'Accounting')}
+            enabled={staged.accounting}
+            onToggle={(next) => setFlag('accounting', next)}
+          />
+
           <FeatureCard
             icon={Briefcase}
             title={t('settings.features.hoursLogging.title', 'Hours logging')}