perf(frontend): set loading immediately on playlist switch for instant indicator

packages/frontend/src/hooks/usePaginatedSongs.ts

@@ -168,6 +168,8 @@ export const usePaginatedSongs = (options: UsePaginatedSongsOptions = {}) => {
     currentSearchQueryRef.current = searchQuery;
 
     // Clear state for instant visual feedback
+    loadingRef.current = true;
+    setLoading(true);
     setSongs([]);
     setTotalSongs(0);
     setTotalDuration(undefined);