fix: Placement 스캔 전 mpa(piezo power on) 전송 — ADC 미수신 해결

기기가 piezo power off 상태에서 maa 보내면 reb 없이 raa만 옴
→ performScan()에서 sendPiezoPowerOn() 호출 추가

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/java/com/example/medilightv2android/ui/views/monitoring/PlacementGuideView.kt

@@ -305,6 +305,7 @@ fun PlacementGuideView(appState: AppState) {
             fun performScan() {
                 if (isScanning) return
                 isScanning = true
+                bleManager.sendPiezoPowerOn()
                 bleManager.piezoCollector.onMultiChannelComplete = { channels ->
                     isScanning = false
                     scanCount++