refactor: remove LocaleSwitcher component and update locale handling

- Removed LocaleSwitcher from NavUser, SiteHeader, and WorkbenchHeader components.
- Updated tests to reflect the removal of LocaleSwitcher.
- Changed default locale from "en-US" to "zh-CN" in i18n configuration.
- Refactored locale resolution logic to read configured locale without relying on browser language detection.
- Updated AgentDesk SDK to support language configuration from public API.
- Cleaned up unused auth options fetching in the auth API.

internal/pkg/errorsx/errors.go

@@ -86,7 +86,7 @@ func (e *I18nError) Error() string {
 	if e == nil {
 		return ""
 	}
-	return e.Message(i18nx.LocaleZhCN)
+	return e.Message(i18nx.DefaultLocale)
 }
 
 func (e *I18nError) Unwrap() error {