feat(player): report the real position, and skip an unplayable track

The seek bar was a setInterval counter reconciled only on track
change: measured 3 s behind during steady playback and 30 s behind
after four keyboard seeks, because the seek shortcut never told it.
And `loadCurrentTrack`/`playCurrentTrack` logged, returned false and
emitted nothing, so double-clicking a moved file did nothing, twice,
forever — while auto-advance onto a bad file stopped playback dead.

- A 1 Hz position ticker while playing, plus an immediate report on
  load, play, pause, seek and natural finish. The payload carries a
  `trackChangeId` (the store is a singleton, so a bar mounting later
  must not adopt a report about the previous track) and a `seq` (the
  same second reported twice still has to reset interpolation).
- `PlaybackFailed` from both failure paths, and `playCurrentOrSkip`
  steps over tracks that will not load — bounded by the queue length,
  so a disconnected drive stops after one pass instead of spinning
  through a RepeatAll wrap. `PlayIndex` still reverts: the user picked
  that track.
- `SeekFailed` is emitted when the seek itself fails, not only when
  nothing is loaded, and is followed by a position report so the
  optimistic move is taken back by the mechanism that fixed the drift.
- A queue that simply ran out no longer unloads the player, so the
  finished track stays on the bar at 0:00.

backend/queue/handlers.go

@@ -29,18 +29,20 @@ func (q *Queue) OnPlaybackFinished() {
 	nextIdx := q.nextIndex()
 	if nextIdx == -1 {
 		// Queue exhausted — this is the extension point for a future fallback playlist.
-		q.onQueueExhausted()
+		q.onQueueExhausted(false)
 		q.mu.Unlock()
 		q.recordPlay(finishedID)
 
 		return
 	}
 
-	prevIndex := q.currentIndex
 	q.currentIndex = nextIdx
 
-	if !q.playCurrentTrack() {
-		q.currentIndex = prevIndex
+	// Skip over tracks that cannot be played instead of reverting.
+	// Reverting stopped playback dead on the first moved file and left
+	// Next unable to get past it, since Next hit the same track.
+	if !q.playCurrentOrSkip(true, q.nextIndex) {
+		q.onQueueExhausted(false)
 		q.mu.Unlock()
 		q.recordPlay(finishedID)