fix: defer MPV playback start until user unpauses on session restore

Instead of play → pause → sleep → seek when restoring a paused session,
introduce loadTrackPaused which sets internal state and fires UI/OS
callbacks (song change, paused) without touching MPV. On Continue(),
detect pendingLoadPaused and start MPV playback from the saved position.

Also triggers waveform pre-generation for the paused track via the
onBeforeSongChange hook.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/playbackcommands.go

@@ -30,6 +30,8 @@ const (
 	cmdLoadRadioStation // arg: *mediaprovider.RadioStation, arg2: InsertQueueMode
 
 	cmdForceRestartPlayback
+
+	cmdLoadTrackPaused // arg: int (idx), arg2: float64 (startTime)
 )
 
 type playbackCommand struct {
@@ -185,6 +187,17 @@ func (c *playbackCommandQueue) SetQueueState(tracks []*mediaprovider.Track, queu
 	c.cmdAvailable.Signal()
 }
 
+func (c *playbackCommandQueue) LoadTrackPaused(idx int, startTime float64) {
+	c.mutex.Lock()
+	c.queue = append(c.queue, playbackCommand{
+		Type: cmdLoadTrackPaused,
+		Arg:  idx,
+		Arg2: startTime,
+	})
+	c.mutex.Unlock()
+	c.cmdAvailable.Signal()
+}
+
 func (c *playbackCommandQueue) addCommand(command playbackCommand) {
 	c.mutex.Lock()
 	c.queue = append(c.queue, command)