feat(users): 添加用户语言偏好字段

- 在 User 模型中添加 language 字段，默认值为 'zh-CN'
- 更新 UpdateUserDto 以支持语言偏好参数
- 在用户更新服务中实现语言偏好的保存逻辑

src/users/users.service.ts

@@ -222,7 +222,7 @@ export class UsersService {
 
   // 更新用户昵称、头像
   async updateUser(updateUserDto: UpdateUserDto, userId: string): Promise<UpdateUserResponseDto> {
-    const { name, avatar, gender, birthDate, dailyStepsGoal, dailyCaloriesGoal, pilatesPurposes, weight, initialWeight, targetWeight, height, activityLevel } = updateUserDto;
+    const { name, avatar, gender, birthDate, language, dailyStepsGoal, dailyCaloriesGoal, pilatesPurposes, weight, initialWeight, targetWeight, height, activityLevel } = updateUserDto;
 
     this.logger.log(`updateUser: ${JSON.stringify(updateUserDto, null, 2)}`);
 
@@ -252,6 +252,10 @@ export class UsersService {
       user.birthDate = dayjs(birthDate * 1000).startOf('day').toDate();
       userChanges.birthDate = birthDate;
     }
+    if (language) {
+      user.language = language;
+      userChanges.language = language;
+    }
 
     this.logger.log(`updateUser user: ${JSON.stringify(user, null, 2)}`);