fix: auto-play next song — set ffmpeg=null on close, handle async playNext

Two bugs prevented auto-advance:
1. FFmpeg close handler was empty — ChildProcess object persisted after
   close, so frame loop check `!this.ffmpeg` never triggered trackEnd
2. playNext() is async (resolves URL) but trackEnd handler didn't catch
   rejected promises, causing silent failures

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/audio/player.ts

@@ -79,7 +79,7 @@ export class AudioPlayer extends EventEmitter {
     });
 
     this.ffmpeg.on("close", () => {
-      // Let frame loop drain remaining buffer
+      this.ffmpeg = null; // Signal frame loop that no more data is coming
     });
 
     this.ffmpeg.on("error", (err) => {