Add admin hooks, components, and tests for statistics, navigation, and access control

- Introduced `useAdminStats`, `useAdminUsers`, and `useAdminOrganizations` hooks for admin data fetching with React Query.
- Added `AdminSidebar`, `Breadcrumbs`, and related navigation components for the admin section.
- Implemented comprehensive unit and integration tests for admin components.
- Created E2E tests for admin access control, navigation, and dashboard functionality.
- Updated exports to include new admin components.

frontend/src/components/admin/index.ts

@@ -1,4 +1,6 @@
 // Admin-specific components
-// Examples: UserTable, OrganizationForm, StatisticsCard, etc.
 
-export {};
+export { AdminSidebar } from './AdminSidebar';
+export { Breadcrumbs } from './Breadcrumbs';
+export { StatCard } from './StatCard';
+export { DashboardStats } from './DashboardStats';