fix: mpa 첫 스캔 시 1회만 전송 (매초 중복 전송 제거)

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

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

@@ -301,11 +301,16 @@ fun PlacementGuideView(appState: AppState) {
 
             Spacer(modifier = Modifier.height(24.dp))
 
-            // Scan function (reusable)
+            // 첫 스캔 시 1회만 piezo power on
+            var piezoPowered by remember { mutableStateOf(false) }
+
             fun performScan() {
                 if (isScanning) return
                 isScanning = true
-                bleManager.sendPiezoPowerOn()
+                if (!piezoPowered) {
+                    bleManager.sendPiezoPowerOn()
+                    piezoPowered = true
+                }
                 bleManager.piezoCollector.onMultiChannelComplete = { channels ->
                     isScanning = false
                     scanCount++