feat(usage-query): decouple credentials from provider config

Add independent credential fields for usage query to support different
query endpoints and authentication methods.

Changes:
- Add `apiKey` and `baseUrl` fields to UsageScript struct
- Remove dependency on provider config credentials in query_usage
- Update test_usage_script to accept independent credential parameters
- Add credential input fields in UsageScriptModal based on template:
  * General: apiKey + baseUrl
  * NewAPI: baseUrl + accessToken + userId
  * Custom: no additional fields (full freedom)
- Auto-clear irrelevant fields when switching templates
- Add i18n text for "credentialsConfig"

Benefits:
- Query API can use different endpoint/key than provider config
- Better separation of concerns
- More flexible for various usage query scenarios

src/i18n/locales/zh.json

@@ -356,6 +356,7 @@
     "templateCustom": "自定义",
     "templateGeneral": "通用模板",
     "templateNewAPI": "NewAPI",
+    "credentialsConfig": "凭证配置",
     "accessToken": "访问令牌",
     "accessTokenPlaceholder": "在'安全设置'里生成",
     "userId": "用户 ID",