fix(frontend): handle voice PCM data as JSON instead of binary

- Add onVoicePcmData and onVoiceActiveUser handlers to WebSocket interface
- Add voice_pcm_data and voice_active_user event cases in socket message handler
- Update useAudioPlayback to decode base64 PCM from JSON format
- Connect onVoicePcmData to audio.handleIncomingPcm in App.tsx

Format change:
- Old: Binary (4 bytes userId + PCM buffer)
- New: JSON {userId: string, pcm: base64string}

This matches the format sent by discord-gateway via Redis.
Now PCM audio will flow correctly: Discord → Gateway → Redis → Backend → WebSocket → Browser!

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/frontend/src/App.tsx

@@ -76,7 +76,7 @@ export default function App() {
   );
 
   const socket = useDashboardSocket({
-    onBinary: audio.handleIncomingPcm,
+    onVoicePcmData: (d) => audio.handleIncomingPcm(d as { userId: string; pcm: string }),
     onUserState: (users) => setActiveSpeakers(users as ActiveSpeaker[]),
     onMessageCreated: (m) =>
       messages.setMessages((prev) => mergeMessages(prev, [m as MessageRecord])),