feat: setup barrelsby for automatic barrel file generation

Add barrelsby as a dev dependency with a `generate-barrels` npm script and a `barrelesby.json` config targeting `./src`. This automates creation of barrel/index files to simplify imports across the project.

src/data/models/chat/sync_message.ts

@@ -0,0 +1,41 @@
+/**
+ * 待同步的单条消息
+ * 原始 Dart: SyncMessage (lib/data/models/chat/chat_sync.dart)
+ */
+export class SyncMessage {
+  readonly role: string;
+  readonly content: string;
+  readonly timestamp: string;
+
+  constructor(params: { role: string; content: string; timestamp: string }) {
+    this.role = params.role;
+    this.content = params.content;
+    this.timestamp = params.timestamp;
+    Object.freeze(this);
+  }
+
+  toJson(): Record<string, unknown> {
+    return {
+      role: this.role,
+      content: this.content,
+      timestamp: this.timestamp,
+    };
+  }
+
+  static fromJson(json: Record<string, unknown>): SyncMessage {
+    const requireString = (key: string): string => {
+      const v = json[key];
+      if (typeof v !== "string") {
+        throw new Error(
+          `SyncMessage.${key} is required and must be a string`
+        );
+      }
+      return v;
+    };
+    return new SyncMessage({
+      role: requireString("role"),
+      content: requireString("content"),
+      timestamp: requireString("timestamp"),
+    });
+  }
+}