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/entity/UserInfo.java

@@ -14,9 +14,6 @@ public class UserInfo {
     @Column(nullable = false, unique = true, length = 64)
     private String openId;
     
-    @Column(length = 50)
-    private String nickname;
-    
     @Column(length = 255)
     private String avatarUrl;
     
@@ -63,9 +60,6 @@ public class UserInfo {
     public String getOpenId() { return openId; }
     public void setOpenId(String openId) { this.openId = openId; }
     
-    public String getNickname() { return nickname; }
-    public void setNickname(String nickname) { this.nickname = nickname; }
-    
     public String getAvatarUrl() { return avatarUrl; }
     public void setAvatarUrl(String avatarUrl) { this.avatarUrl = avatarUrl; }