feat: 新增聊天消息关键词搜索功能

- wcdbCore: 绑定 wcdb_search_messages DLL 函数，添加 searchMessages 方法
- wcdbWorker: 添加 searchMessages case
- wcdbService: 添加 searchMessages 代理方法
- chatService: 添加 searchMessages，结果解析为 Message 对象
- main: 注册 chat:searchMessages IPC handler

electron/services/wcdbService.ts

@@ -406,6 +406,10 @@ export class WcdbService {
     return this.callWorker('getMessageById', { sessionId, localId })
   }
 
+  async searchMessages(keyword: string, sessionId?: string, limit?: number, offset?: number, beginTimestamp?: number, endTimestamp?: number): Promise<{ success: boolean; messages?: any[]; error?: string }> {
+    return this.callWorker('searchMessages', { keyword, sessionId, limit, offset, beginTimestamp, endTimestamp })
+  }
+
   /**
    * 获取语音数据
    */