fix: resolve subscription UX issues and add placeholder pages

- Profile: show "未订阅" card when subscription inactive/0 days
- Profile: subscription management navigates to page instead of modal
- Subscribe-plans: data-driven plan cards with mock payment flow
- Subscribe-plans: show current subscription status at top
- Subscribe-prompt: "先使用普通模式" goes to wear-check directly
- Add help and contact placeholder pages
- Fix unbindDevice to work without explicit deviceId

miniprogram/pages/contact/contact.wxml

@@ -0,0 +1,9 @@
+<view class="page">
+  <view class="page-header" 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>
+</view>