Phase 4.0: WebSocket chat and danmaku system implemented

frontend/lib/services/chat_service.dart

@@ -0,0 +1,61 @@
+import 'dart:async';
+import 'dart:convert';
+import 'package:web_socket_channel/web_socket_channel.dart';
+
+class ChatMessage {
+  final String type;
+  final String username;
+  final String content;
+  final String roomId;
+
+  ChatMessage({required this.type, required this.username, required this.content, required this.roomId});
+
+  factory ChatMessage.fromJson(Map<String, dynamic> json) {
+    return ChatMessage(
+      type: json['type'] ?? 'chat',
+      username: json['username'] ?? 'Anonymous',
+      content: json['content'] ?? '',
+      roomId: json['room_id'] ?? '',
+    );
+  }
+
+  Map<String, dynamic> toJson() => {
+    'type': type,
+    'username': username,
+    'content': content,
+    'room_id': roomId,
+  };
+}
+
+class ChatService {
+  WebSocketChannel? _channel;
+  final StreamController<ChatMessage> _messageController = StreamController<ChatMessage>.broadcast();
+
+  Stream<ChatMessage> get messages => _messageController.stream;
+
+  void connect(String baseUrl, String roomId, String username) {
+    final wsUri = Uri.parse(baseUrl).replace(scheme: 'ws', path: '/api/ws/room/$roomId', queryParameters: {'username': username});
+    _channel = WebSocketChannel.connect(wsUri);
+
+    _channel!.stream.listen((data) {
+      final json = jsonDecode(data);
+      _messageController.add(ChatMessage.fromJson(json));
+    }, onError: (err) {
+      print("[WS ERROR] $err");
+    }, onDone: () {
+      print("[WS DONE] Connection closed");
+    });
+  }
+
+  void sendMessage(String content, String username, String roomId, {String type = 'chat'}) {
+    if (_channel != null) {
+      final msg = ChatMessage(type: type, username: username, content: content, roomId: roomId);
+      _channel!.sink.add(jsonEncode(msg.toJson()));
+    }
+  }
+
+  void dispose() {
+    _channel?.sink.close();
+    _messageController.close();
+  }
+}