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/utils/api.js

@@ -14,7 +14,7 @@ module.exports = {
   bindDevice: function (deviceId) { return http.post('/api/v1/device/bind', { device_id: deviceId }) },
   confirmBind: function (deviceId, token) { return http.post('/api/v1/device/bind/confirm', { device_id: deviceId, bind_token: token }) },
   mockBind: function (deviceId) { return http.post('/api/v1/device/mock-bind', { device_id: deviceId }) },
-  unbindDevice: function (deviceId) { return http.post('/api/v1/device/unbind', { device_id: deviceId }) },
+  unbindDevice: function (deviceId) { return http.post('/api/v1/device/unbind', deviceId ? { device_id: deviceId } : {}) },
 
   // Subscription
   getSubscription: function () { return http.get('/api/v1/subscription') },