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/emit.go

@@ -43,6 +43,28 @@ func (q *Queue) emitModeChanged() {
 	)
 }
 
+// emitPlaybackFailed tells the frontend that a track could not be
+// played.  Before this existed the failure was logged, the index was
+// reverted and nothing reached the user: a moved file was a button
+// that did nothing, twice, forever (errors.C1).
+func (q *Queue) emitPlaybackFailed(track Track, reason error) {
+	msg := ""
+	if reason != nil {
+		msg = reason.Error()
+	}
+
+	events.Emit(
+		q.ctx,
+		events.PlaybackFailed,
+		PlaybackFailure{
+			FilePath: track.FilePath,
+			Title:    track.Title,
+			Artist:   track.Artist,
+			Reason:   msg,
+		},
+	)
+}
+
 // emitTracksModified emits a delta update for track list changes.
 func (q *Queue) emitTracksModified(
 	action string,