feat: implement WebCodecs Opus decode for browser listen

- Add broadcastOpusToWeb to PcmBroadcaster interface for raw Opus packets
- Server broadcasts Opus frames with mode byte (1) + user hash + packet data
- Browser detects packet mode: mode=1 for Opus, mode=0 for legacy PCM
- Implement WebCodecs AudioDecoder for Opus decoding in browser
- Keep existing PCM playback as fallback for compatibility
- Show error if WebCodecs unsupported
- Fixes listen feature under Bun where native Opus decode unavailable

src/types.ts

@@ -42,6 +42,7 @@ export interface SegmentMetadata extends UserMetadata {
 
 export interface PcmBroadcaster {
   broadcastPcmToWeb?: (chunk: Buffer, userId: string) => void;
+  broadcastOpusToWeb?: (chunk: Buffer, userId: string) => void;
   updateActiveUser?: (
     userId: string,
     data: { username: string; avatar: string; speaking: boolean },