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/PlayerModal.tsx

@@ -203,8 +203,15 @@ export default function PlayerModal({
 }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
-  // The active item is queue[index] (falls back to the single opened video).
+  // Track the playing item by id (not a frozen index) so it survives the queue changing
-  const [index, setIndex] = useState(startIndex ?? 0);
+  // under us — e.g. an item removed in another tab. The index is derived from the *live*
+  // queue, so the N / M counter and prev/next neighbours stay correct without disrupting
+  // playback of the current video.
+  const [playingId, setPlayingId] = useState<string>(
+    queue?.[startIndex ?? 0]?.id ?? video.id
+  );
+  let index = queue ? queue.findIndex((v) => v.id === playingId) : 0;
+  if (index < 0) index = 0;
   const active = queue && queue[index] ? queue[index] : video;
   const hasQueue = !!queue && queue.length > 1;
   // startAt only applies to the item we opened with; advanced items resume their own pos.
@@ -369,7 +376,7 @@ export default function PlayerModal({
                 autoMarkedRef.current = true;
                 setWatched(true);
               }
-              if (queue && index < queue.length - 1) setIndex(index + 1);
+              if (queue && index < queue.length - 1) setPlayingId(queue[index + 1].id);
             }
           },
         },
@@ -418,7 +425,7 @@ export default function PlayerModal({
         {hasQueue && (
           <div className="flex items-center justify-center gap-5 px-4 py-2 border-b border-border text-sm">
             <button
-              onClick={() => setIndex((i) => Math.max(0, i - 1))}
+              onClick={() => index > 0 && setPlayingId(queue![index - 1].id)}
               disabled={index === 0}
               className="inline-flex items-center gap-1.5 text-muted enabled:hover:text-fg disabled:opacity-40 transition"
             >
@@ -428,7 +435,7 @@ export default function PlayerModal({
               {index + 1} / {queue!.length}
             </span>
             <button
-              onClick={() => setIndex((i) => Math.min(queue!.length - 1, i + 1))}
+              onClick={() => index < queue!.length - 1 && setPlayingId(queue![index + 1].id)}
               disabled={index === queue!.length - 1}
               className="inline-flex items-center gap-1.5 text-muted enabled:hover:text-fg disabled:opacity-40 transition"
             >

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;