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

i18n/i18n.go

@@ -0,0 +1,176 @@
+package i18n
+
+import (
+	"embed"
+	"strings"
+	"sync"
+
+	"github.com/gin-gonic/gin"
+	"github.com/nicksnyder/go-i18n/v2/i18n"
+	"golang.org/x/text/language"
+	"gopkg.in/yaml.v3"
+
+	"github.com/QuantumNous/new-api/common"
+	"github.com/QuantumNous/new-api/constant"
+)
+
+const (
+	LangZh      = "zh"
+	LangEn      = "en"
+	DefaultLang = LangEn // Fallback to English if language not supported
+)
+
+//go:embed locales/*.yaml
+var localeFS embed.FS
+
+var (
+	bundle     *i18n.Bundle
+	localizers = make(map[string]*i18n.Localizer)
+	mu         sync.RWMutex
+	initOnce   sync.Once
+)
+
+// Init initializes the i18n bundle and loads all translation files
+func Init() error {
+	var initErr error
+	initOnce.Do(func() {
+		bundle = i18n.NewBundle(language.Chinese)
+		bundle.RegisterUnmarshalFunc("yaml", yaml.Unmarshal)
+
+		// Load embedded translation files
+		files := []string{"locales/zh.yaml", "locales/en.yaml"}
+		for _, file := range files {
+			_, err := bundle.LoadMessageFileFS(localeFS, file)
+			if err != nil {
+				initErr = err
+				return
+			}
+		}
+
+		// Pre-create localizers for supported languages
+		localizers[LangZh] = i18n.NewLocalizer(bundle, LangZh)
+		localizers[LangEn] = i18n.NewLocalizer(bundle, LangEn)
+
+		// Set the TranslateMessage function in common package
+		common.TranslateMessage = T
+	})
+	return initErr
+}
+
+// GetLocalizer returns a localizer for the specified language
+func GetLocalizer(lang string) *i18n.Localizer {
+	lang = normalizeLang(lang)
+
+	mu.RLock()
+	loc, ok := localizers[lang]
+	mu.RUnlock()
+
+	if ok {
+		return loc
+	}
+
+	// Create new localizer for unknown language (fallback to default)
+	mu.Lock()
+	defer mu.Unlock()
+
+	// Double-check after acquiring write lock
+	if loc, ok = localizers[lang]; ok {
+		return loc
+	}
+
+	loc = i18n.NewLocalizer(bundle, lang, DefaultLang)
+	localizers[lang] = loc
+	return loc
+}
+
+// T translates a message key using the language from gin context
+func T(c *gin.Context, key string, args ...map[string]any) string {
+	lang := GetLangFromContext(c)
+	return Translate(lang, key, args...)
+}
+
+// Translate translates a message key for the specified language
+func Translate(lang, key string, args ...map[string]any) string {
+	loc := GetLocalizer(lang)
+
+	config := &i18n.LocalizeConfig{
+		MessageID: key,
+	}
+
+	if len(args) > 0 && args[0] != nil {
+		config.TemplateData = args[0]
+	}
+
+	msg, err := loc.Localize(config)
+	if err != nil {
+		// Return key as fallback if translation not found
+		return key
+	}
+	return msg
+}
+
+// GetLangFromContext extracts the language setting from gin context
+func GetLangFromContext(c *gin.Context) string {
+	if c == nil {
+		return DefaultLang
+	}
+
+	// Try to get language from context (set by middleware)
+	if lang := c.GetString(string(constant.ContextKeyLanguage)); lang != "" {
+		return normalizeLang(lang)
+	}
+
+	return DefaultLang
+}
+
+// ParseAcceptLanguage parses the Accept-Language header and returns the preferred language
+func ParseAcceptLanguage(header string) string {
+	if header == "" {
+		return DefaultLang
+	}
+
+	// Simple parsing: take the first language tag
+	parts := strings.Split(header, ",")
+	if len(parts) == 0 {
+		return DefaultLang
+	}
+
+	// Get the first language and remove quality value
+	firstLang := strings.TrimSpace(parts[0])
+	if idx := strings.Index(firstLang, ";"); idx > 0 {
+		firstLang = firstLang[:idx]
+	}
+
+	return normalizeLang(firstLang)
+}
+
+// normalizeLang normalizes language code to supported format
+func normalizeLang(lang string) string {
+	lang = strings.ToLower(strings.TrimSpace(lang))
+
+	// Handle common variations
+	switch {
+	case strings.HasPrefix(lang, "zh"):
+		return LangZh
+	case strings.HasPrefix(lang, "en"):
+		return LangEn
+	default:
+		return DefaultLang
+	}
+}
+
+// SupportedLanguages returns a list of supported language codes
+func SupportedLanguages() []string {
+	return []string{LangZh, LangEn}
+}
+
+// IsSupported checks if a language code is supported
+func IsSupported(lang string) bool {
+	lang = normalizeLang(lang)
+	for _, supported := range SupportedLanguages() {
+		if lang == supported {
+			return true
+		}
+	}
+	return false
+}