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/layout.tsx

@@ -24,13 +24,19 @@ export default function AdminLayout({
 }) {
   return (
     <AuthGuard requireAdmin>
+      <a
+        href="#main-content"
+        className="sr-only focus:not-sr-only focus:absolute focus:left-4 focus:top-4 focus:z-50 focus:rounded-md focus:bg-primary focus:px-4 focus:py-2 focus:text-primary-foreground focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2"
+      >
+        Skip to main content
+      </a>
       <div className="flex min-h-screen flex-col">
         <Header />
         <div className="flex flex-1">
           <AdminSidebar />
           <div className="flex flex-1 flex-col">
             <Breadcrumbs />
-            <main className="flex-1 overflow-y-auto">
+            <main id="main-content" className="flex-1 overflow-y-auto">
               {children}
             </main>
           </div>