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/detail/detail.wxml

@@ -1,10 +1,16 @@
-<activity-board
-  wx:if="{{activity}}"
-  activity="{{activity}}"
-  show-past="{{false}}"
-  bind:map="openMap"
-  bind:signup="goSignup"
-/>
+<block wx:if="{{activity}}">
+  <activity-board
+    activity="{{activity}}"
+    show-past="{{false}}"
+    bind:map="openMap"
+    bind:signup="openSignup"
+  />
+  <signup-sheet
+    show="{{signupOpen}}"
+    activity-id="{{activity.id}}"
+    bind:close="closeSignup"
+  />
+</block>
 <view wx:elif="{{failed}}" class="empty">
   <text>活动不存在或已下线</text>
   <view class="home" bindtap="goHome">回到首页</view>