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/hooks/useDebounce.ts

@@ -0,0 +1,28 @@
+'use client';
+
+import { useState, useEffect } from 'react';
+
+/**
+ * Debounce a value by a given delay.
+ *
+ * @param value  The raw (fast-changing) value, e.g. a search input string.
+ * @param delay  Debounce delay in milliseconds (default 400ms).
+ * @returns      The debounced value – only updates after the user stops
+ *               changing `value` for `delay` ms.
+ *
+ * @example
+ * const [search, setSearch] = useState('');
+ * const debouncedSearch = useDebounce(search, 400);
+ *
+ * // use `debouncedSearch` in your query key / API call
+ */
+export function useDebounce<T>(value: T, delay = 400): T {
+  const [debounced, setDebounced] = useState<T>(value);
+
+  useEffect(() => {
+    const timer = setTimeout(() => setDebounced(value), delay);
+    return () => clearTimeout(timer);
+  }, [value, delay]);
+
+  return debounced;
+}