fix: subscription extend instead of overwrite, improve placeholder pages

- Subscription purchase now extends expire_time when user has active
  subscription, instead of cancelling and replacing
- Admin subscription creation uses same extend logic
- Subscribe page shows "续费" button and extend message for renewals
- Help and contact pages: add pink header, centered icon + text

miniprogram/pages/help/help.wxml

@@ -1,9 +1,13 @@
 <view class="page">
-  <view class="page-header" style="padding-top: {{statusBarHeight + 24}}px;">
+  <view class="page-header page-header-pink" style="padding-top: {{statusBarHeight + 24}}px;">
     <view class="nav-back" bindtap="onBack">‹ 返回</view>
     <view class="page-header-title">使用帮助</view>
   </view>
   <view class="page-content">
-    <view class="placeholder">内容建设中...</view>
+    <view class="placeholder-container">
+      <view class="placeholder-icon">📖</view>
+      <view class="placeholder-text">内容建设中</view>
+      <view class="placeholder-sub">帮助文档正在编写，敬请期待</view>
+    </view>
   </view>
 </view>