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/ErrorBoundary.tsx

@@ -0,0 +1,32 @@
+import React from 'react';
+
+interface State {
+  error: Error | null;
+}
+
+export class ErrorBoundary extends React.Component<React.PropsWithChildren, State> {
+  state: State = { error: null };
+
+  static getDerivedStateFromError(error: Error) {
+    return { error };
+  }
+
+  render() {
+    if (this.state.error) {
+      return (
+        <div style={{ padding: 40, color: '#ef4444', background: '#0f0f0f', height: '100vh', fontFamily: 'monospace' }}>
+          <h1 style={{ fontSize: 24, marginBottom: 16 }}>App Error</h1>
+          <pre style={{ whiteSpace: 'pre-wrap', color: '#f59e0b' }}>{this.state.error.message}</pre>
+          <pre style={{ whiteSpace: 'pre-wrap', color: '#9ca3af', marginTop: 8, fontSize: 12 }}>{this.state.error.stack}</pre>
+          <button
+            onClick={() => window.location.reload()}
+            style={{ marginTop: 20, padding: '8px 16px', background: '#3b82f6', color: '#fff', border: 'none', borderRadius: 8, cursor: 'pointer' }}
+          >
+            Reload
+          </button>
+        </div>
+      );
+    }
+    return this.props.children;
+  }
+}