feat(playlists): keep the player queue live across tabs

Track the playing item by id instead of a frozen index, deriving the index from the
live queue so the N / M counter and prev/next neighbours stay correct when the playlist
changes underneath (e.g. an item removed in another tab) without disrupting playback of
the current video. Refetch the playlist list/detail on window focus so such changes flow
in, updating the player's queue length in near-real-time.

frontend/src/components/Playlists.tsx

@@ -108,7 +108,11 @@ export default function Playlists() {
   // The open player, as an index into `items` (the queue); null = closed.
   const [playingIndex, setPlayingIndex] = useState<number | null>(null);
 
-  const listQuery = useQuery({ queryKey: ["playlists"], queryFn: () => api.playlists() });
+  const listQuery = useQuery({
+    queryKey: ["playlists"],
+    queryFn: () => api.playlists(),
+    refetchOnWindowFocus: true,
+  });
   const playlists = listQuery.data ?? [];
 
   // Keep the selection valid: default to the first playlist, and if the selected one
@@ -127,6 +131,9 @@ export default function Playlists() {
     queryKey: ["playlist", selectedId],
     queryFn: () => api.playlist(selectedId as number),
     enabled: selectedId != null,
+    // Refetch when returning to the tab so a change made elsewhere (another tab/device)
+    // shows up — including the player's live N / M count, which reads the queue length.
+    refetchOnWindowFocus: true,
   });
   const detail = detailQuery.data;