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

main.go

@@ -14,6 +14,7 @@ import (
 	"github.com/QuantumNous/new-api/common"
 	"github.com/QuantumNous/new-api/constant"
 	"github.com/QuantumNous/new-api/controller"
+	"github.com/QuantumNous/new-api/i18n"
 	"github.com/QuantumNous/new-api/logger"
 	"github.com/QuantumNous/new-api/middleware"
 	"github.com/QuantumNous/new-api/model"
@@ -151,6 +152,7 @@ func main() {
 	//server.Use(gzip.Gzip(gzip.DefaultCompression))
 	server.Use(middleware.RequestId())
 	server.Use(middleware.PoweredBy())
+	server.Use(middleware.I18n())
 	middleware.SetUpLogger(server)
 	// Initialize session store
 	store := cookie.NewStore([]byte(common.SessionSecret))
@@ -278,5 +280,14 @@ func InitResources() error {
 	// 启动系统监控
 	common.StartSystemMonitor()
 
+	// Initialize i18n
+	err = i18n.Init()
+	if err != nil {
+		common.SysError("failed to initialize i18n: " + err.Error())
+		// Don't return error, i18n is not critical
+	} else {
+		common.SysLog("i18n initialized with languages: " + strings.Join(i18n.SupportedLanguages(), ", "))
+	}
+
 	return nil
 }