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/wcdbWorker.ts

@@ -140,6 +140,9 @@ if (parentPort) {
                 case 'getMessageById':
                     result = await core.getMessageById(payload.sessionId, payload.localId)
                     break
+                case 'searchMessages':
+                    result = await core.searchMessages(payload.keyword, payload.sessionId, payload.limit, payload.offset, payload.beginTimestamp, payload.endTimestamp)
+                    break
                 case 'getVoiceData':
                     result = await core.getVoiceData(payload.sessionId, payload.createTime, payload.candidates, payload.localId, payload.svrId)
                     if (!result.success) {