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/pages/past/past.wxml

@@ -4,6 +4,11 @@
     <view class="btn del {{deleting ? 'active' : ''}}" bindtap="onDelete">{{deleting ? '完成' : '删除'}}</view>
   </view>
 
+  <view class="section-head">
+    <view class="section-bar"></view>
+    <text class="section-title">往期活动</text>
+  </view>
+
   <view wx:if="{{loaded && !list.length}}" class="empty">还没有往期活动</view>
 
   <view