feat(i18n): add backend multi-language support with user language preference

- 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

dto/user_settings.go

@@ -14,6 +14,7 @@ type UserSetting struct {
 	RecordIpLog           bool    `json:"record_ip_log,omitempty"`                  // 是否记录请求和错误日志IP
 	SidebarModules        string  `json:"sidebar_modules,omitempty"`                // SidebarModules 左侧边栏模块配置
 	BillingPreference     string  `json:"billing_preference,omitempty"`             // BillingPreference 扣费策略（订阅/钱包）
+	Language              string  `json:"language,omitempty"`                       // Language 用户语言偏好 (zh, en)
 }
 
 var (