feat(i18n): 实现用户语言偏好服务器同步功能

- 添加 UserLanguage 类型定义 ('zh-CN' | 'en-US')
- 在 UpdateUserDto 中新增 language 字段
- 实现语言切换时自动同步到服务器
- 为已登录用户保存语言偏好设置
- 服务器同步失败时降级处理，不影响本地语言切换

app/(tabs)/personal.tsx

@@ -9,6 +9,7 @@ import { useAppDispatch, useAppSelector } from '@/hooks/redux';
 import { useAuthGuard } from '@/hooks/useAuthGuard';
 import type { BadgeDto } from '@/services/badges';
 import { reportBadgeShowcaseDisplayed } from '@/services/badges';
+import { updateUser, type UserLanguage } from '@/services/users';
 import { fetchAvailableBadges, selectBadgeCounts, selectBadgePreview, selectSortedBadges } from '@/store/badgesSlice';
 import { selectActiveMembershipPlanName } from '@/store/membershipSlice';
 import { DEFAULT_MEMBER_NAME, fetchActivityHistory, fetchMyProfile } from '@/store/userSlice';
@@ -89,13 +90,33 @@ export default function PersonalScreen() {
     }
     try {
       setIsSwitchingLanguage(true);
+      
+      // 将 AppLanguage ('zh' | 'en') 映射到 UserLanguage ('zh-CN' | 'en-US')
+      const languageMap: Record<AppLanguage, UserLanguage> = {
+        'zh': 'zh-CN',
+        'en': 'en-US',
+      };
+      const userLanguage = languageMap[language];
+      
+      // 先切换本地语言
       await changeAppLanguage(language);
+      
+      // 如果用户已登录，同步更新服务器语言设置
+      if (isLoggedIn) {
+        try {
+          await updateUser({ language: userLanguage });
+          log.info('语言设置已同步到服务器', { language: userLanguage });
+        } catch (error) {
+          log.warn('同步语言设置到服务器失败', error);
+          // 服务器更新失败不影响本地语言切换，静默处理
+        }
+      }
     } catch (error) {
       log.warn('语言切换失败', error);
     } finally {
       setIsSwitchingLanguage(false);
     }
-  }, [activeLanguageCode, isSwitchingLanguage]);
+  }, [activeLanguageCode, isSwitchingLanguage, isLoggedIn]);
 
   // 推送通知设置仅在独立页面管理
 

services/users.ts

@@ -2,6 +2,9 @@ import { api } from '@/services/api';
 
 export type Gender = 'male' | 'female';
 
+// 用户语言设置
+export type UserLanguage = 'zh-CN' | 'en-US';
+
 export type UpdateUserDto = {
   name?: string;
   avatar?: string; // base64 字符串
@@ -21,6 +24,7 @@ export type UpdateUserDto = {
   armCircumference?: number; // 臂围
   thighCircumference?: number; // 大腿围
   calfCircumference?: number; // 小腿围
+  language?: UserLanguage; // 用户语言偏好
 };
 
 export async function updateUser(dto: UpdateUserDto): Promise<Record<string, any>> {