refactor(config): 合并用户注册配置并调整默认配额

将 UserRegisterProperties 内嵌到 AppConfig，删除独立配置类；
freeTrialQuota 由 5 改为 3，新增 rewardBalance 字段；
同步更新 UserServiceImpl 初始化逻辑及 yml 配置。

src/main/resources/application-dev.yml

@@ -69,21 +69,6 @@ dromara:
         domain: https://resource.loveamorkey.com/ # ????????/???????https://abcd.s3.ap-east-1.amazonaws.com/
         base-path: avatar/ # ????
 
-
-mailgun:
-  api-key: ${MAILGUN_API_KEY}          # ?? Private API Key
-  domain: sandboxxxxxxx.mailgun.org    # ?????????
-  from-email: no-reply@yourdomain.com  # ?????
-  from-name: Key Of Love             # ?????????
-
-# ??????
-user:
-  register:
-    # ?????????????
-    free-trial-quota: 5
-
-
-
 ############## Sa-Token ?? (??: https://sa-token.cc) ##############
 sa-token:
   # token ??????? cookie ???