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

frontend/src/components/grid/CameraGrid.tsx

@@ -4,7 +4,7 @@ import { CameraGridCell } from './CameraGridCell';
 const STAGGER_MS = 200;
 
 export function CameraGrid() {
-  const cameras = useConfigStore((s) => s.enabledCameras());
+  const cameras = useConfigStore((s) => s.cameras);
   const gridConfig = useConfigStore((s) => s.config?.grid);
 
   const count = cameras.length;