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/DashboardStats.tsx

@@ -0,0 +1,63 @@
+/**
+ * DashboardStats Component
+ * Displays admin dashboard statistics in stat cards
+ */
+
+'use client';
+
+import { useAdminStats } from '@/lib/api/hooks/useAdmin';
+import { StatCard } from './StatCard';
+import { Users, UserCheck, Building2, Activity } from 'lucide-react';
+import { Alert, AlertDescription } from '@/components/ui/alert';
+import { AlertCircle } from 'lucide-react';
+
+export function DashboardStats() {
+  const { data: stats, isLoading, isError, error } = useAdminStats();
+
+  if (isError) {
+    return (
+      <Alert variant="destructive">
+        <AlertCircle className="h-4 w-4" />
+        <AlertDescription>
+          Failed to load dashboard statistics: {error?.message || 'Unknown error'}
+        </AlertDescription>
+      </Alert>
+    );
+  }
+
+  return (
+    <div
+      className="grid gap-4 md:grid-cols-2 lg:grid-cols-4"
+      data-testid="dashboard-stats"
+    >
+      <StatCard
+        title="Total Users"
+        value={stats?.totalUsers ?? 0}
+        icon={Users}
+        description="All registered users"
+        loading={isLoading}
+      />
+      <StatCard
+        title="Active Users"
+        value={stats?.activeUsers ?? 0}
+        icon={UserCheck}
+        description="Users with active status"
+        loading={isLoading}
+      />
+      <StatCard
+        title="Organizations"
+        value={stats?.totalOrganizations ?? 0}
+        icon={Building2}
+        description="Total organizations"
+        loading={isLoading}
+      />
+      <StatCard
+        title="Active Sessions"
+        value={stats?.totalSessions ?? 0}
+        icon={Activity}
+        description="Current active sessions"
+        loading={isLoading}
+      />
+    </div>
+  );
+}