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/help/help.js

@@ -0,0 +1,18 @@
+Page({
+  data: {
+    statusBarHeight: 44
+  },
+
+  onLoad: function () {
+    var app = getApp()
+    this.setData({ statusBarHeight: app.globalData.statusBarHeight })
+  },
+
+  onBack: function () {
+    wx.navigateBack({
+      fail: function () {
+        wx.switchTab({ url: '/pages/profile/profile' })
+      }
+    })
+  }
+})