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/app/admin/layout.tsx

@@ -1,12 +1,14 @@
 /**
  * Admin Route Group Layout
  * Wraps all admin routes with AuthGuard requiring superuser privileges
+ * Includes sidebar navigation and breadcrumbs
  */
 
 import type { Metadata } from 'next';
 import { AuthGuard } from '@/components/auth';
 import { Header } from '@/components/layout/Header';
 import { Footer } from '@/components/layout/Footer';
+import { AdminSidebar, Breadcrumbs } from '@/components/admin';
 
 export const metadata: Metadata = {
   title: {
@@ -24,9 +26,15 @@ export default function AdminLayout({
     <AuthGuard requireAdmin>
       <div className="flex min-h-screen flex-col">
         <Header />
-        <main className="flex-1">
-          {children}
-        </main>
+        <div className="flex flex-1">
+          <AdminSidebar />
+          <div className="flex flex-1 flex-col">
+            <Breadcrumbs />
+            <main className="flex-1 overflow-y-auto">
+              {children}
+            </main>
+          </div>
+        </div>
         <Footer />
       </div>
     </AuthGuard>