Add web HTTP-FLV playback path

frontend/lib/widgets/web_stream_player_stub.dart

@@ -0,0 +1,16 @@
+import 'package:flutter/material.dart';
+
+class WebStreamPlayer extends StatelessWidget {
+  final String streamUrl;
+
+  const WebStreamPlayer({super.key, required this.streamUrl});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Text(
+      'Web playback is unavailable on this platform.',
+      style: TextStyle(color: Colors.white),
+      textAlign: TextAlign.center,
+    );
+  }
+}