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.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>