fix(quick-14): add roll-back-on-failure to queue index advancement

- Next() rolls back currentIndex and skips emitIndexChanged on load failure
- Previous() applies same pattern to all three branches (RepeatOne, restart, navigate)
- OnPlaybackFinished() rolls back currentIndex on playCurrentTrack failure
- PlayIndex() and playFromStart() also guard against load failures
- RepeatOne paths guard emitIndexChanged with the bool return value

backend/queue/handlers.go

@@ -12,8 +12,9 @@ func (q *Queue) OnPlaybackFinished() {
 
 	// Repeat One: replay the current track.
 	if q.repeatMode == RepeatOne {
-		q.playCurrentTrack()
-		q.emitIndexChanged()
+		if q.playCurrentTrack() {
+			q.emitIndexChanged()
+		}
 
 		return
 	}
@@ -26,7 +27,14 @@ func (q *Queue) OnPlaybackFinished() {
 		return
 	}
 
+	prevIndex := q.currentIndex
 	q.currentIndex = nextIdx
-	q.playCurrentTrack()
+
+	if !q.playCurrentTrack() {
+		q.currentIndex = prevIndex
+
+		return
+	}
+
 	q.emitIndexChanged()
 }