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

middleware/i18n.go

@@ -0,0 +1,50 @@
+package middleware
+
+import (
+	"github.com/gin-gonic/gin"
+
+	"github.com/QuantumNous/new-api/common"
+	"github.com/QuantumNous/new-api/constant"
+	"github.com/QuantumNous/new-api/dto"
+	"github.com/QuantumNous/new-api/i18n"
+)
+
+// I18n middleware detects and sets the language preference for the request
+func I18n() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		lang := detectLanguage(c)
+		c.Set(string(constant.ContextKeyLanguage), lang)
+		c.Next()
+	}
+}
+
+// detectLanguage determines the language preference for the request
+// Priority: 1. User setting (if logged in) -> 2. Accept-Language header -> 3. Default language
+func detectLanguage(c *gin.Context) string {
+	// 1. Try to get language from user setting (set by auth middleware)
+	if userSetting, ok := common.GetContextKeyType[dto.UserSetting](c, constant.ContextKeyUserSetting); ok {
+		if userSetting.Language != "" && i18n.IsSupported(userSetting.Language) {
+			return userSetting.Language
+		}
+	}
+
+	// 2. Parse Accept-Language header
+	acceptLang := c.GetHeader("Accept-Language")
+	if acceptLang != "" {
+		lang := i18n.ParseAcceptLanguage(acceptLang)
+		if i18n.IsSupported(lang) {
+			return lang
+		}
+	}
+
+	// 3. Return default language
+	return i18n.DefaultLang
+}
+
+// GetLanguage returns the current language from gin context
+func GetLanguage(c *gin.Context) string {
+	if lang := c.GetString(string(constant.ContextKeyLanguage)); lang != "" {
+		return lang
+	}
+	return i18n.DefaultLang
+}