修复注册错误提示不明确的问题

backend/app/routers/auth.py

@@ -43,14 +43,14 @@ def register(user_data: UserCreate, db: Session = Depends(get_db)):
         if existing_phone:
             raise HTTPException(
                 status_code=status.HTTP_400_BAD_REQUEST,
-                detail="该手机号已被注册"
+                detail="E00040:该手机号已被注册，请更换手机号"
             )
     if user_data.email:
         existing_email = db.query(User).filter(User.email == user_data.email).first()
         if existing_email:
             raise HTTPException(
                 status_code=status.HTTP_400_BAD_REQUEST,
-                detail="邮箱已被注册"
+                detail="E00041:该邮箱已被注册，请更换邮箱"
             )
     
     # 创建用户