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;
 }

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>

miniprogram/pages/help/help.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;
 }

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>