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/Breadcrumbs.tsx

@@ -0,0 +1,92 @@
+/**
+ * Admin Breadcrumbs
+ * Displays navigation breadcrumb trail for admin pages
+ */
+
+'use client';
+
+import Link from 'next/link';
+import { usePathname } from 'next/navigation';
+import { ChevronRight } from 'lucide-react';
+
+interface BreadcrumbItem {
+  label: string;
+  href: string;
+}
+
+const pathLabels: Record<string, string> = {
+  admin: 'Admin',
+  users: 'Users',
+  organizations: 'Organizations',
+  settings: 'Settings',
+};
+
+export function Breadcrumbs() {
+  const pathname = usePathname();
+
+  // Generate breadcrumb items from pathname
+  const generateBreadcrumbs = (): BreadcrumbItem[] => {
+    const segments = pathname.split('/').filter(Boolean);
+    const breadcrumbs: BreadcrumbItem[] = [];
+
+    let currentPath = '';
+    segments.forEach((segment) => {
+      currentPath += `/${segment}`;
+      const label = pathLabels[segment] || segment;
+      breadcrumbs.push({
+        label,
+        href: currentPath,
+      });
+    });
+
+    return breadcrumbs;
+  };
+
+  const breadcrumbs = generateBreadcrumbs();
+
+  if (breadcrumbs.length === 0) {
+    return null;
+  }
+
+  return (
+    <nav
+      aria-label="Breadcrumb"
+      className="border-b bg-background px-6 py-3"
+      data-testid="breadcrumbs"
+    >
+      <ol className="flex items-center space-x-2 text-sm">
+        {breadcrumbs.map((breadcrumb, index) => {
+          const isLast = index === breadcrumbs.length - 1;
+
+          return (
+            <li key={breadcrumb.href} className="flex items-center">
+              {index > 0 && (
+                <ChevronRight
+                  className="mx-2 h-4 w-4 text-muted-foreground"
+                  aria-hidden="true"
+                />
+              )}
+              {isLast ? (
+                <span
+                  className="font-medium text-foreground"
+                  aria-current="page"
+                  data-testid={`breadcrumb-${breadcrumb.label.toLowerCase()}`}
+                >
+                  {breadcrumb.label}
+                </span>
+              ) : (
+                <Link
+                  href={breadcrumb.href}
+                  className="text-muted-foreground hover:text-foreground transition-colors"
+                  data-testid={`breadcrumb-${breadcrumb.label.toLowerCase()}`}
+                >
+                  {breadcrumb.label}
+                </Link>
+              )}
+            </li>
+          );
+        })}
+      </ol>
+    </nav>
+  );
+}