fix: handle DatabaseType enum conversion in config management

Fix type conversion issues when handling DatabaseType enum values in
configuration loading and GUI updates.

- config/loader.py: Handle both enum and string types for DB_TYPE
- config/schema.py: Properly convert db_type when it's already a string
- gui/config_manager.py: Add Enum type conversion with validation in set_nested_value

These changes prevent AttributeError when accessing .value on string values
and ensure proper type handling throughout the configuration system.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

gui/config_manager.py

@@ -94,8 +94,20 @@ class ConfigManager:
         for k in keys[:-1]:
             obj = getattr(obj, k)
 
+        # 获取目标字段信息用于类型转换
+        target_field = keys[-1]
+        field_type = type(getattr(obj, target_field))
+
+        # 如果是字符串且目标字段是枚举类型，进行转换
+        if isinstance(value, str) and hasattr(field_type, "__members__"):  # 它是一个 Enum
+            try:
+                value = field_type(value)
+            except ValueError:
+                # 无效的枚举值，保持当前值不变
+                value = getattr(obj, target_field)
+
         # 设置最终值
-        setattr(obj, keys[-1], value)
+        setattr(obj, target_field, value)
 
     def reset_to_defaults(self) -> None:
         """重置为默认配置"""