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/contact/contact.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">联系我们</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">联系方式正在整理，敬请期待</view>
+    </view>
   </view>
 </view>

miniprogram/pages/contact/contact.wxss

@@ -1,6 +1,24 @@
-.placeholder {
-  text-align: center;
+.placeholder-container {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  padding: 120rpx 40rpx;
+}
+
+.placeholder-icon {
+  font-size: 100rpx;
+  margin-bottom: 32rpx;
+}
+
+.placeholder-text {
+  font-size: 32rpx;
+  font-weight: 600;
+  color: #333333;
+  margin-bottom: 12rpx;
+}
+
+.placeholder-sub {
+  font-size: 26rpx;
   color: #999999;
-  font-size: 28rpx;
-  padding: 80rpx 0;
 }