refactor: 删除nickname字段，统一使用username

- UserInfo实体类删除nickname字段
- UserInfoService.loginOrRegister使用username
- UserController删除nickname相关代码
- 前端所有页面统一使用username
- auth-modal.vue登录时使用username

src/main/java/xuan/controller/UserController.java

@@ -77,7 +77,6 @@ public class UserController {
             data.put("id", user.getId());
             data.put("userId", user.getUserId());
             data.put("openId", user.getOpenId());
-            data.put("nickname", user.getNickname());
             data.put("avatarUrl", user.getAvatarUrl());
             data.put("username", user.getUsername());
             data.put("phone", user.getPhone());
@@ -139,7 +138,6 @@ public class UserController {
                 data.put("id", user.getId());
                 data.put("userId", user.getUserId());
                 data.put("openId", user.getOpenId());
-                data.put("nickname", user.getNickname());
                 data.put("avatarUrl", user.getAvatarUrl());
                 data.put("username", user.getUsername());
                 data.put("phone", user.getPhone());