Add admin UX improvements, constants refactor, and comprehensive tests

- Introduced constants for admin hooks: `STATS_FETCH_LIMIT`, `DEFAULT_PAGE_LIMIT`, and `STATS_REFETCH_INTERVAL` to enhance readability and maintainability.
- Updated query guards to ensure data fetching is restricted to superusers.
- Enhanced accessibility across admin components by adding `aria-hidden` attributes and improving focus-visible styles.
- Simplified `useAdminStats`, `useAdminUsers`, and `useAdminOrganizations` with shared constants.
- Added 403 Forbidden page with proper structure, styling, and tests.
- Implemented new tests for admin hooks, DashboardStats, AdminLayout, and ForbiddenPage for better coverage.

frontend/src/app/admin/organizations/page.tsx

@@ -22,8 +22,8 @@ export default function AdminOrganizationsPage() {
         {/* Back Button + Header */}
         <div className="flex items-center gap-4">
           <Link href="/admin">
-            <Button variant="outline" size="icon">
-              <ArrowLeft className="h-4 w-4" />
+            <Button variant="outline" size="icon" aria-label="Back to Admin Dashboard">
+              <ArrowLeft className="h-4 w-4" aria-hidden="true" />
             </Button>
           </Link>
           <div>