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/subscribe-prompt/subscribe-prompt.wxml

@@ -30,6 +30,6 @@
     </view>
 
     <button class="btn-primary btn-primary-gold" bindtap="onViewPlans">立即订阅</button>
-    <button class="btn-secondary" bindtap="onBack">先使用普通模式</button>
+    <button class="btn-secondary" bindtap="onUseFreeMode">先使用普通模式</button>
   </view>
 </view>