Ship signup sheet, subscribe reminders, cover crop, and theme updates.

Wire real WeChat app config, prefer activity cover over posters, and add admin cover cropping for the home banner.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/lib/serialize.js

@@ -24,7 +24,8 @@ function publicActivity(activity, posters) {
     theme: activity.theme || 0,
     cover: activity.cover || '',
     video: activity.video || '',
-    poster: (poster && poster.image) || activity.cover || '',
+    // Activity cover wins over linked posters so admin “封面图片” updates show immediately.
+    poster: activity.cover || (poster && poster.image) || '',
     createdAt: activity.createdAt
   }
 }