fix: address audit findings for Phase 1 scan

- Extract _cleanup() to properly unregister ADC listener + clear timers
- ADC callback unregisters itself on first receive (prevent duplicates)
- setParams failure now stops scanning and clears all state
- Timeout handler unregisters ADC listener
- Add BLE connection check before scan
- SCAN_TIMEOUT 8s → 12s (must exceed hold_time 10s)
- Pre-compute barWidth in JS instead of float math in WXML
- Error only shows when scanning is false

miniprogram/pages/auto-scan/auto-scan.wxml

@@ -23,7 +23,7 @@
           <view class="pd-region">{{item.region}}</view>
           <view class="pd-value {{item.pd > 0 ? '' : 'pd-empty'}}">{{item.pdText}}</view>
           <view class="pd-bar-bg" wx:if="{{item.pd > 0}}">
-            <view class="pd-bar-fill" style="width: {{item.pd > 4095 ? 100 : item.pd / 40.95}}%"></view>
+            <view class="pd-bar-fill" style="width: {{item.barWidth}}%"></view>
           </view>
         </view>
       </view>
@@ -32,7 +32,7 @@
       </view>
     </view>
 
-    <view class="scan-error" wx:if="{{error}}">{{error}}</view>
+    <view class="scan-error" wx:if="{{!scanning && error}}">{{error}}</view>
 
     <button class="btn-primary mt-30" wx:if="{{!scanning}}" bindtap="onNext">下一步</button>
   </view>