fix: resolve critical race conditions and high-severity bugs

- Add isAdvancing guard to playNext() to prevent concurrent calls from
  skipping songs (trackEnd + user !next race condition)
- Replace removeAllListeners in WebSocket setup with tracked named
  listeners, attach only once per bot instead of every 5 seconds
- Add .catch() to unhandled playNext() in cmdVote
- Add sessionId counter to AudioPlayer to discard stale setTimeout
  callbacks after stop()+play() transitions
- Defer nulling TS3 client until disconnect() promise resolves
- Add null checks for providers in play-by-id and add-by-id endpoints
- Add PCM buffer backpressure (pause FFmpeg at 960KB, resume at 480KB)
- Change volume slider from @input to @change to avoid flooding server
- Wrap error-reporting sendTextMessage in try/catch to prevent
  unhandled rejection in error handler
- Clamp elapsed getter to song duration
- Log FFmpeg stderr at debug level instead of swallowing
- Return null from prev() in Sequential mode instead of wrapping

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

web/src/stores/player.ts

@@ -69,9 +69,10 @@ export const usePlayerStore = defineStore('player', {
     /** Interpolated elapsed: serverElapsed + time since last sync (if playing) */
     elapsed(): number {
       if (!this.activeBot?.currentSong) return 0;
-      if (!this.wasPlaying || this.serverSyncTime === 0) return this.serverElapsed;
-      if (this.isPaused) return this.serverElapsed;
-      return this.serverElapsed + (Date.now() - this.serverSyncTime) / 1000;
+      const maxDuration = this.activeBot.currentSong.duration || Infinity;
+      if (!this.wasPlaying || this.serverSyncTime === 0) return Math.min(this.serverElapsed, maxDuration);
+      if (this.isPaused) return Math.min(this.serverElapsed, maxDuration);
+      return Math.min(this.serverElapsed + (Date.now() - this.serverSyncTime) / 1000, maxDuration);
     },
   },