fix: 修复新用户金币默认值为3

- 在@PrePersist中设置coins默认值为3
- 确保新用户注册时获得3个金币

src/main/java/xuan/entity/UserInfo.java

@@ -46,6 +46,9 @@ public class UserInfo {
         createTime = LocalDateTime.now();
         updateTime = createTime;
         vipLevel = 0;
+        if (coins == null) {
+            coins = 3;
+        }
         streakDays = 0;
     }