Fix streaming: MSE-first with go2rtc init protocol

- Switch from WebRTC-first to MSE-first streaming (more reliable
  across all camera types including high-res IP cameras)
- Send required {"type":"mse"} init message to go2rtc WebSocket
- Fix infinite re-render loop in configStore (pre-compute enabled
  cameras instead of deriving in selector)
- Fix mqtt_bridge global variable scope in broadcast()
- Add React ErrorBoundary for visible crash reporting
- Remove unused go2rtcUrl dependency from useStream hook

backend/mqtt_bridge.py

@@ -23,6 +23,7 @@ def unregister_ws_client(ws):
 
 
 async def broadcast(message: dict):
+    global _ws_clients
     data = json.dumps(message)
     disconnected = set()
     for ws in _ws_clients:
@@ -30,7 +31,8 @@ async def broadcast(message: dict):
             await ws.send_text(data)
         except Exception:
             disconnected.add(ws)
-    _ws_clients -= disconnected
+    if disconnected:
+        _ws_clients -= disconnected
 
 
 async def mqtt_listener():