refactor: add useDebounce hook and replace manual debounce in admin search

- Create reusable useDebounce<T> hook in frontend/src/hooks/useDebounce.ts
- Replace manual setTimeout/clearTimeout + extra state in AdminAppsPage
  with clean useDebounce(search, 400) pattern
- Removes timer state and handleSearch wrapper function

frontend/src/app/dashboard/admin/apps/page.tsx

@@ -8,6 +8,7 @@ import { toast } from 'react-toastify';
 import type { Application } from '@/types';
 import { Search, X, Package, Hexagon, User, Database, Box } from 'lucide-react';
 import { useConfirm } from '@/components/confirm-modal';
+import { useDebounce } from '@/hooks/useDebounce';
 
 const statusColors: Record<string, string> = {
   running: 'badge-green',
@@ -23,15 +24,7 @@ export default function AdminAppsPage() {
   const queryClient = useQueryClient();
   const confirm = useConfirm();
   const [search, setSearch] = useState('');
-  const [debouncedSearch, setDebouncedSearch] = useState('');
-  const [timer, setTimer] = useState<NodeJS.Timeout | null>(null);
-
-  const handleSearch = (value: string) => {
-    setSearch(value);
-    if (timer) clearTimeout(timer);
-    const t = setTimeout(() => setDebouncedSearch(value), 400);
-    setTimer(t);
-  };
+  const debouncedSearch = useDebounce(search, 400);
 
   const { data: apps = [], isLoading } = useQuery<Application[]>({
     queryKey: ['admin-applications', debouncedSearch],
@@ -138,12 +131,12 @@ export default function AdminAppsPage() {
           type="text"
           placeholder="Search by app name, user name, email, or user ID…"
           value={search}
-          onChange={(e) => handleSearch(e.target.value)}
+          onChange={(e) => setSearch(e.target.value)}
           className="input pl-10 w-full"
         />
         {search && (
           <button
-            onClick={() => { setSearch(''); setDebouncedSearch(''); }}
+            onClick={() => setSearch('')}
             className="absolute right-3 top-1/2 -translate-y-1/2 text-gray-400 hover:text-gray-600"
           >
             <X className="w-4 h-4" />