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/subscribe-plans/subscribe-plans.js

@@ -118,9 +118,16 @@ Page({
       return
     }
 
+    var isRenew = self.data.subscription && self.data.subscription.status === 'active' && self.data.subRemaining > 0
+    var planDays = plan.key === 'yearly' ? 365 : 30
+    var title = isRenew ? '确认续费' : '确认支付'
+    var content = isRenew
+      ? '在现有订阅基础上延长' + planDays + '天，模拟支付 ¥' + plan.price + '？（测试环境）'
+      : '模拟支付 ¥' + plan.price + '？（测试环境）'
+
     wx.showModal({
-      title: '确认支付',
-      content: '模拟支付 ¥' + plan.price + '？（测试环境）',
+      title: title,
+      content: content,
       success: function (res) {
         if (res.confirm) {
           self.doPurchase(plan)

miniprogram/pages/subscribe-plans/subscribe-plans.wxml

@@ -38,7 +38,7 @@
     </view>
 
     <button class="btn-primary btn-primary-gold" bindtap="onPurchase" disabled="{{purchasing}}" loading="{{purchasing}}">
-      确认支付
+      {{subscription && subscription.status === 'active' && subRemaining > 0 ? '续费' : '确认支付'}}
     </button>
     <view class="agreement">支付即表示同意《订阅协议》</view>
   </view>