refactor: 统一变量名为username

- login方法中将nickname变量改名为username
- 保持前端传入参数名不变(nickname)
- 后端统一使用username变量名

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

@@ -38,7 +38,7 @@ public class UserController {
         Map<String, Object> result = new HashMap<>();
         try {
             String code = params.get("code") != null ? params.get("code").toString() : null;
-            String nickname = params.get("nickname") != null ? params.get("nickname").toString() : "微信用户";
+            String username = params.get("nickname") != null ? params.get("nickname").toString() : "微信用户";
             String avatarUrl = params.get("avatarUrl") != null ? params.get("avatarUrl").toString() : "";
             
             String openId = null;
@@ -71,7 +71,7 @@ public class UserController {
                 localAvatarUrl = downloadAvatar(avatarUrl, openId);
             }
             
-            UserInfo user = userInfoService.loginOrRegister(openId, nickname, localAvatarUrl);
+            UserInfo user = userInfoService.loginOrRegister(openId, username, localAvatarUrl);
             
             Map<String, Object> data = new HashMap<>();
             data.put("id", user.getId());