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

miniprogram/pages/index/index.wxml

@@ -23,6 +23,15 @@
       </view>
     </view>
 
+    <view class="sub-info" bindtap="onViewSubscription">
+      <view class="sub-info-left">
+        <text class="sub-info-icon">💳</text>
+        <text wx:if="{{subscription && subscription.status === 'active'}}">智能模式 · 剩余{{subRemaining}}天</text>
+        <text wx:else>未订阅智能模式</text>
+      </view>
+      <text class="sub-info-arrow">›</text>
+    </view>
+
     <button class="btn-primary" bindtap="onStartTreatment">开始护理</button>
     <button class="btn-secondary" bindtap="onManageDevice">设备管理</button>
   </view>