feat(uniapp_v2): gate sign-in on 签到广告 article view

Before triggering setSignIntegral, fetch the random required article;
when one is returned, show a fullscreen overlay with title, image, and
rich-text content (loaded via getArticleDetails) plus a 10s countdown
gating the confirm button. Falls through to direct sign-in when the
endpoint returns null or fails.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pro_v3.5.1/view/uniapp_v2/api/user.js

@@ -157,6 +157,14 @@ export function getSignCalendar(data) {
 	return request.get('sign/calendar', data)
 }
 
+/**
+ * 签到前置阅读：随机返回"签到广告"分类下一条文章
+ * 后端无该分类或分类下没有可用文章时 data 为 null，前端可跳过门槛直接签到
+ */
+export function getSignRequiredArticle() {
+	return request.get('sign/required_article')
+}
+
 /**
  * 活动状态
  *