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>

config/loader.py

@@ -111,7 +111,7 @@ class ConfigLoader:
             "ERP_IGNORE_HTTPS_ERRORS": config.erp.ignore_https_errors,
             "ERP_AUTO_CLOSE_BROWSER": config.erp.auto_close_browser,
             # 数据库配置
-            "DB_TYPE": config.database.db_type.value,
+            "DB_TYPE": config.database.db_type.value if isinstance(config.database.db_type, DatabaseType) else config.database.db_type,
             "DB_SERVER": config.database.server,
             "DB_NAME": config.database.database,
             "DB_USERNAME": config.database.username,

config/schema.py

@@ -312,7 +312,7 @@ class AppConfig:
                 "auto_close_browser": self.erp.auto_close_browser,
             },
             "database": {
-                "db_type": self.database.db_type.value,
+                "db_type": self.database.db_type if isinstance(self.database.db_type, str) else self.database.db_type.value,
                 "server": self.database.server,
                 "database": self.database.database,
                 "username": self.database.username,