首次提交：初始化项目

lib/models/message.dart

@@ -0,0 +1,28 @@
+/// Message model representing a chat message
+class Message {
+  final String id;
+  final String content;
+  final bool isUser;
+  final DateTime timestamp;
+
+  Message({
+    required this.id,
+    required this.content,
+    required this.isUser,
+    DateTime? timestamp,
+  }) : timestamp = timestamp ?? DateTime.now();
+
+  Message copyWith({
+    String? id,
+    String? content,
+    bool? isUser,
+    DateTime? timestamp,
+  }) {
+    return Message(
+      id: id ?? this.id,
+      content: content ?? this.content,
+      isUser: isUser ?? this.isUser,
+      timestamp: timestamp ?? this.timestamp,
+    );
+  }
+}