fix(queue): prevent ghost next_track after context switch

Auto-advance from a previous track could arrive after the user
switched to a new album/playlist, advancing the new queue's index
and skipping a track (observed: Sorceress track skipped).

Added queue epoch counter that increments on setQueue/clearQueue.
nextTrackGuarded() captures epoch before the invoke and discards
the result if the queue changed while in-flight. Applied to all
auto-advance paths (track-end, gapless transition) but not to
manual skip (user-initiated).

cc

src/lib/stores/queueStore.ts

@@ -368,11 +368,24 @@ export async function addTracksToQueueNext(tracks: BackendQueueTrack[]): Promise
   }
 }
 
+/**
+ * Queue epoch counter — incremented on every setQueue/clearQueue call.
+ * Used by nextTrackGuarded() to discard stale auto-advance results
+ * that arrive after a context switch (album/playlist change).
+ */
+let queueEpoch = 0;
+
+/** Current queue epoch (read-only for callers). */
+export function getQueueEpoch(): number {
+  return queueEpoch;
+}
+
 /**
  * Set queue with new tracks (V2)
  */
 export async function setQueue(tracks: BackendQueueTrack[], startIndex: number, clearLocal = true): Promise<boolean> {
   try {
+    queueEpoch++;
     await invoke('v2_set_queue', { tracks, startIndex });
     if (clearLocal) {
       localTrackIds = new Set();
@@ -390,6 +403,7 @@ export async function setQueue(tracks: BackendQueueTrack[], startIndex: number,
  */
 export async function clearQueue(): Promise<boolean> {
   try {
+    queueEpoch++;
     await invoke('v2_clear_queue');
     return true;
   } catch (err) {
@@ -422,6 +436,20 @@ export async function nextTrack(): Promise<BackendQueueTrack | null> {
   }
 }
 
+/**
+ * Get next track, but discard the result if the queue changed while
+ * the invoke was in-flight (prevents ghost auto-advance after context switch).
+ */
+export async function nextTrackGuarded(): Promise<BackendQueueTrack | null> {
+  const epochBefore = queueEpoch;
+  const result = await nextTrack();
+  if (queueEpoch !== epochBefore) {
+    console.warn('[Queue] Discarding stale next_track result (queue changed during invoke)');
+    return null;
+  }
+  return result;
+}
+
 /**
  * Get previous track from queue (V2)
  */

src/routes/+page.svelte

@@ -221,6 +221,7 @@
     clearQueue,
     playQueueIndex,
     nextTrack,
+    nextTrackGuarded,
     previousTrack,
     moveQueueTrack,
     setLocalTrackIds,
@@ -4369,7 +4370,7 @@
         return;
       }
       const previousTrackId = currentTrack?.id ?? null;
-      const nextTrackResult = await nextTrack();
+      const nextTrackResult = await nextTrackGuarded();
       if (nextTrackResult) {
         // Defensive fallback for issue #80:
         // if backend returns same track on auto-advance while repeat-one is off,
@@ -4383,7 +4384,7 @@
             '[Player] Auto-advance returned same track id, forcing one extra nextTrack()',
             previousTrackId
           );
-          const forcedNext = await nextTrack();
+          const forcedNext = await nextTrackGuarded();
           if (forcedNext && forcedNext.id !== previousTrackId) {
             await playQueueTrack(forcedNext);
             return;
@@ -4496,7 +4497,7 @@
       if (qconnectSuppressLocalPlaybackAutomation) return;
       console.log('[Gapless] Handling transition to track', trackId);
       // Advance the queue to match backend state
-      const advanced = await nextTrack();
+      const advanced = await nextTrackGuarded();
       if (advanced && advanced.id === trackId) {
         // Queue advanced successfully — update UI metadata
         await playQueueTrack(advanced, undefined, true);