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.wxss

@@ -0,0 +1,6 @@
+.placeholder {
+  text-align: center;
+  color: #999999;
+  font-size: 28rpx;
+  padding: 80rpx 0;
+}