feat: complete feature gaps across all modules

Server:
- Add settings-cache with 60s TTL for feature toggle checks
- Enforce maintenance_mode on login, enable_binding on device bind

Admin console:
- Remove dead "发送通知" button from user detail
- Firmware check calls real API and compares versions

Miniprogram:
- Wear-check: dynamic battery/connected from BLE state
- Login: hide non-functional phone auth button
- Treating: show actual selected regions instead of hardcoded "全脸"
- Index: display subscription status with tap to manage
- Auto-scan: show "待检测" instead of "--" for PD data
- Agreements: tap shows "内容建设中" modal

server/src/lib/settings-cache.js

@@ -0,0 +1,14 @@
+const settingsDao = require('../dao/settings.dao')
+
+let cache = null
+let cacheTime = 0
+const TTL = 60000 // 1 minute
+
+async function getSettings() {
+  if (cache && Date.now() - cacheTime < TTL) return cache
+  cache = await settingsDao.getAll()
+  cacheTime = Date.now()
+  return cache
+}
+
+module.exports = { getSettings }