feat(phase6): video grid, noise suppression, whisper system

- New VideoGrid component: live camera feeds (local + remote) in VoicePanel
- Camera toggle in VoiceControls: enable/disable video per participant
- Noise suppression toggle in VoiceControls + voice store
- VOICE_WHISPER WS event: backend routes whisper to target user only
- Whisper UI: per-participant whisper button, notification toasts with dismiss
- Fix circular import between voice.ts and ws.ts (use lazy accessor)

internal/gateway/events.go

@@ -22,6 +22,7 @@ const (
 	EventVoiceLeave     = "VOICE_LEAVE"
 	EventVoiceMute      = "VOICE_MUTE"
 	EventVoiceDeafen    = "VOICE_DEAFEN"
+	EventVoiceWhisper   = "VOICE_WHISPER"
 )
 
 // Event represents a WebSocket event sent to clients.