The i18n middleware runs before UserAuth, so user settings weren't
available when language was detected. Now GetLangFromContext checks
user settings first (set by UserAuth) before falling back to the
language set by middleware or Accept-Language header.
- Add go-i18n library for internationalization
- Create i18n package with translation keys and YAML locale files (zh/en)
- Implement i18n middleware for language detection from user settings and Accept-Language header
- Add Language field to UserSetting DTO
- Update API response helpers with i18n support (ApiErrorI18n, ApiSuccessI18n)
- Migrate hardcoded messages in token, redemption, and user controllers
- Add frontend language preference settings component
- Sync language preference across header selector and user settings
- Auto-restore user language preference on login
