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>

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>

frontend/src/app/admin/page.tsx

@@ -39,7 +39,7 @@ export default function AdminPage() {
             <Link href="/admin/users" className="block">
               <div className="rounded-lg border bg-card p-6 transition-colors hover:bg-accent cursor-pointer">
                 <div className="flex items-center gap-3 mb-2">
-                  <Users className="h-5 w-5 text-primary" />
+                  <Users className="h-5 w-5 text-primary" aria-hidden="true" />
                   <h3 className="font-semibold">User Management</h3>
                 </div>
                 <p className="text-sm text-muted-foreground">
@@ -51,7 +51,7 @@ export default function AdminPage() {
             <Link href="/admin/organizations" className="block">
               <div className="rounded-lg border bg-card p-6 transition-colors hover:bg-accent cursor-pointer">
                 <div className="flex items-center gap-3 mb-2">
-                  <Building2 className="h-5 w-5 text-primary" />
+                  <Building2 className="h-5 w-5 text-primary" aria-hidden="true" />
                   <h3 className="font-semibold">Organizations</h3>
                 </div>
                 <p className="text-sm text-muted-foreground">
@@ -63,7 +63,7 @@ export default function AdminPage() {
             <Link href="/admin/settings" className="block">
               <div className="rounded-lg border bg-card p-6 transition-colors hover:bg-accent cursor-pointer">
                 <div className="flex items-center gap-3 mb-2">
-                  <Settings className="h-5 w-5 text-primary" />
+                  <Settings className="h-5 w-5 text-primary" aria-hidden="true" />
                   <h3 className="font-semibold">System Settings</h3>
                 </div>
                 <p className="text-sm text-muted-foreground">

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

@@ -22,8 +22,8 @@ export default function AdminSettingsPage() {
         {/* 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>