fix(playlists): don't drop the restored selection during initial load

The auto-select effect ran while the playlist query was still loading (empty list),
nulling the localStorage-restored selection so F5 fell back to the first playlist.
Wait for the first load (listQuery.data) before adjusting the selection.

frontend/src/components/Playlists.tsx

@@ -229,6 +229,9 @@ export default function Playlists({ canWrite }: { canWrite: boolean }) {
   // Keep the selection valid: default to the first playlist, and if the selected one
   // disappears (e.g. just deleted) drop to the next available, or clear when none remain.
   useEffect(() => {
+    // Wait for the first load before adjusting the selection — otherwise the empty
+    // placeholder list would null the restored selection and we'd fall back to the first.
+    if (!listQuery.data) return;
     if (!playlists.length) {
       if (selectedId != null) setSelectedId(null);
       return;
@@ -236,7 +239,7 @@ export default function Playlists({ canWrite }: { canWrite: boolean }) {
     if (selectedId == null || !playlists.some((p) => p.id === selectedId)) {
       setSelectedId(playlists[0].id);
     }
-  }, [playlists, selectedId]);
+  }, [playlists, selectedId, listQuery.data]);
 
   const detailQuery = useQuery({
     queryKey: ["playlist", selectedId],