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>

miniprogram/utils/activity.js

@@ -23,7 +23,7 @@ function goSignup(activity) {
 function shareMessage(activity, fallbackId) {
   const item = activity || {}
   const openid = wx.getStorageSync('openid') || ''
-  const cover = item.poster || item.cover
+  const cover = item.cover || item.poster
   const message = {
     title: item.title || '卓米会',
     path: '/pages/detail/detail?id=' + (item.id || fallbackId || '') + '&sharer=' + openid