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/hub.go

@@ -227,6 +227,28 @@ func (h *Hub) ClientCount() int {
 	return len(h.clients)
 }
 
+// SendToUser sends an event only to the specified user's connected client(s).
+func (h *Hub) SendToUser(userID string, event Event) {
+	data, err := json.Marshal(event)
+	if err != nil {
+		h.logger.Error("failed to marshal event", "type", event.Type, "error", err)
+		return
+	}
+
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+
+	for client := range h.clients {
+		if client.UserID == userID {
+			select {
+			case client.send <- data:
+			default:
+				go func(c *Client) { h.unregister <- c }(client)
+			}
+		}
+	}
+}
+
 // RefreshUserServers loads the server membership for a user from the database
 // and caches it. Call when a client connects or when server membership changes.
 func (h *Hub) RefreshUserServers(userID string) {