T07b: frontend feature pages — companies + contacts + settings + audit + dashboard + search

- 11 new feature pages (CompaniesList/Detail/Form, ContactsList/Detail/Form,
  SettingsProfile/Roles/Users, AuditLog, GlobalSearchResults)
- 3 page updates (Dashboard with StatCard+ActivityFeed, Settings with tree nav+Outlet,
  TopBar with SearchDropdown)
- 13 new routes in routes/index.tsx
- i18n updates (de.json + en.json) with companies/contacts/settings/audit/search keys
- 12 new test files + 2 existing test fixes (TopBar, AppShell)
- 7 shared components (DataGrid, Tabs, SearchDropdown, CsvImportDialog, StatCard,
  ActivityFeed, UnsavedChangesGuard)
- 16 new API hooks in hooks.ts
- Verification: 141 tests pass, build succeeds, tsc --noEmit clean

frontend/src/__tests__/shell/AppShell.test.tsx

@@ -7,6 +7,7 @@ import { AppShell } from '@/components/layout/AppShell';
 vi.mock('@/api/hooks', () => ({
   useLogout: () => ({ mutateAsync: vi.fn(), isPending: false }),
   useSwitchTenant: () => ({ mutateAsync: vi.fn(), isPending: false }),
+  useGlobalSearch: () => ({ data: [], isLoading: false }),
 }));
 
 function renderWithRouter(initialPath = '/dashboard') {