feat: 首页配置封面图上传预览，修复系列活动跳转，活动卡片样式优化

frontend/src/components/activity/ActivityCard.jsx

@@ -7,9 +7,8 @@ const { Title } = Typography;
 
 const getImageUrl = (url) => {
   if (!url) return '';
-  if (url.startsWith('http') || url.startsWith('//')) {
-    try { return new URL(url).pathname; } catch { return url; }
-  }
+  // 外部 URL 直接返回，不提取 pathname
+  if (url.startsWith('http') || url.startsWith('//')) return url;
   return url;
 };
 

frontend/src/pages/activity/Detail.jsx

@@ -396,8 +396,7 @@ const ActivityDetail = () => {
 
       {/* Hero Image with LayoutId for shared transition */}
       <div className={styles.detailHeader}>
-        <motion.img
-          layoutId={`activity-card-${id}`}
+        <img
           src={activity.display_banner_url || cleanUrl(activity.banner_url) || activity.cover_image || 'https://images.unsplash.com/photo-1540575467063-178a50c2df87?w=800&h=600&fit=crop'}
           alt={activity.title}
           className={styles.detailImage}