fix: flush BufferedStreamer ring buffer on seek to prevent stale audio

When seeking, the underlying decoder position was updated but the
BufferedStreamer's ring buffer still contained up to 2 seconds of
pre-seek audio data. The speaker would drain this stale buffer
before playing audio from the new position, causing an audible
delay where the old position's audio continued playing.

Add a Flush() method to BufferedStreamer that resets the ring buffer
pointers, and call it in seekLocked() immediately after a successful
seek. This ensures the speaker starts playing from the seeked
position without any stale audio artifact.

backend/player/player.go

@@ -818,6 +818,13 @@ func (p *Player) seekLocked(targetSeconds int) error {
 
 	speaker.Unlock()
 
+	// Flush the read-ahead buffer so the speaker immediately
+	// plays audio from the new position instead of draining
+	// up to 2 seconds of stale pre-seek samples.
+	if p.buffered != nil {
+		p.buffered.Flush()
+	}
+
 	if p.mediaControls != nil {
 		p.mediaControls.NotifySeek(targetSeconds)
 	}