fix: 修改新用户默认金币为3个

- 修改UserInfo实体类@PrePersist中coins默认值为3
- 确保新注册用户获得3个金币

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

@@ -45,7 +45,7 @@ public class UserInfo {
         createTime = LocalDateTime.now();
         updateTime = createTime;
         vipLevel = 0;
-        coins = 10;
+        coins = 3;
         streakDays = 0;
     }