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.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;
 }