fix: improve UX and fix admin console display issues

Miniprogram:
- Add back buttons to all custom-nav pages
- Add debug/mock buttons (BLE connect, wear check, treatment)
  controlled by __DEV__ flag (hidden in prod)

Admin console:
- Fix log page using nonexistent fields (operator_type, target_type)
  now correctly reads admin_id/user_id/action/detail from API
- Fix subscription date fields (started_at→start_time, expired_at→expire_time)
- Wire up subscription detail/extend/renew action buttons
- Wire up device detail "查看完整日志" button
- Add "创建订阅" button to subscription toolbar
- Fix subscription status mapping (2=expired, 3=cancelled)

Docs:
- Add detailed architecture docs for server, miniprogram, admin console

miniprogram/pages/subscribe-success/subscribe-success.js

@@ -5,6 +5,14 @@ Page({
     expiryDate: ''
   },
 
+  onBack: function () {
+    wx.navigateBack({
+      fail: function () {
+        wx.reLaunch({ url: '/pages/index/index' })
+      }
+    })
+  },
+
   onLoad: function (options) {
     var app = getApp()
     this.setData({ statusBarHeight: app.globalData.statusBarHeight })