- Change default language fallback to English instead of Chinese
- Add ErrRedeemFailed typed error for model layer translation
- Migrate remaining hardcoded messages in controller/user.go
- Add translation keys: redeem.failed, user.create_default_token_error, common.uuid_duplicate, common.invalid_input
- 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