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/components/admin/DashboardStats.tsx

@@ -17,7 +17,7 @@ export function DashboardStats() {
   if (isError) {
     return (
       <Alert variant="destructive">
-        <AlertCircle className="h-4 w-4" />
+        <AlertCircle className="h-4 w-4" aria-hidden="true" />
         <AlertDescription>
           Failed to load dashboard statistics: {error?.message || 'Unknown error'}
         </AlertDescription>