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

web/src/i18n/locales/en.json

@@ -2787,6 +2787,12 @@
     "GC execution failed": "GC execution failed",
     "Cache Directory": "Cache Directory",
     "Available": "Available",
-    "输出价格": "Output Price"
+    "输出价格": "Output Price",
+    "偏好设置": "Preferences",
+    "界面语言和其他个人偏好": "Interface language and other personal preferences",
+    "语言偏好": "Language Preference",
+    "选择您的首选界面语言，设置将自动保存并同步到所有设备": "Select your preferred interface language. Settings will be saved automatically and synced across all devices",
+    "语言偏好已保存": "Language preference saved",
+    "提示：语言偏好会同步到您登录的所有设备，并影响API返回的错误消息语言。": "Note: Language preference syncs across all your logged-in devices and affects the language of API error messages."
   }
 }

web/src/i18n/locales/zh.json

@@ -2732,6 +2732,12 @@
     "套餐名称": "套餐名称",
     "应付金额": "应付金额",
     "支付": "支付",
-    "管理员未开启在线支付功能，请联系管理员配置。": "管理员未开启在线支付功能，请联系管理员配置。"
+    "管理员未开启在线支付功能，请联系管理员配置。": "管理员未开启在线支付功能，请联系管理员配置。",
+    "偏好设置": "偏好设置",
+    "界面语言和其他个人偏好": "界面语言和其他个人偏好",
+    "语言偏好": "语言偏好",
+    "选择您的首选界面语言，设置将自动保存并同步到所有设备": "选择您的首选界面语言，设置将自动保存并同步到所有设备",
+    "语言偏好已保存": "语言偏好已保存",
+    "提示：语言偏好会同步到您登录的所有设备，并影响API返回的错误消息语言。": "提示：语言偏好会同步到您登录的所有设备，并影响API返回的错误消息语言。"
   }
 }