fix: 兼容 Web 同源部署地址

frontend/lib/services/chat_service.dart

@@ -45,7 +45,7 @@ class ChatService {
   Stream<ChatMessage> get messages => _messageController.stream;
 
   void connect(String baseUrl, String roomId, String username) {
-    final wsUri = Uri.parse(baseUrl).replace(
+    final wsUri = _webSocketUri(baseUrl).replace(
       scheme: 'ws',
       path: '/api/ws/room/$roomId',
       queryParameters: {'username': username},
@@ -83,6 +83,21 @@ class ChatService {
     }
   }
 
+  Uri _webSocketUri(String baseUrl) {
+    if (baseUrl.isEmpty) {
+      if (kIsWeb) {
+        return Uri.base.replace(
+          scheme: Uri.base.scheme == 'https' ? 'wss' : 'ws',
+        );
+      }
+      return Uri.parse('http://localhost:8080');
+    }
+
+    final uri = Uri.parse(baseUrl);
+    final scheme = uri.scheme == 'https' ? 'wss' : 'ws';
+    return uri.replace(scheme: scheme);
+  }
+
   void dispose() {
     _channel?.sink.close();
     _messageController.close();