fix: config_flow.py 缺少 _LOGGER 定义

添加 logging 导入和 _LOGGER 定义

custom_components/sigmesh_gateway/config_flow.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import logging
 from typing import Any
 
 import voluptuous as vol
@@ -29,6 +30,8 @@ from .const import (
     PROV_TIMEOUT,
 )
 
+_LOGGER = logging.getLogger(__name__)
+
 
 def _get_serial_ports() -> list:
     """获取可用串口列表（在同步线程中执行）."""