fix+UX: BLE 중복수신 수정, peak detection 개선, UI 대폭 정리

BLE:
- GATT 다중연결 race condition 수정 (reconnectConnecting 플래그)
- onCharacteristicChanged API 레벨 분기 (dual callback 방지)
- DeviceScan: isConnected → isServiceReady 전환 (서비스 준비 후 화면전환)
- Placement: mpa/maa 500ms 딜레이 (power on 대기)
- Disconnect & Unbond 기능 추가 (msr? + removeBond + 저장정보 삭제)
- autoConnect 삭제 (HomeView)

Algorithm:
- 후위벽 피크 >= threshold 조건 추가 (false positive 차단)
- low-echo 분석범위 80샘플 제한 (뒤쪽 노이즈 무시)
- urineLen: post-ant-1 → post-ant 수정
- Placement: UrinAI fallback 제거 (PiezoEchoAnalyzer만 사용)
- 기기 프리셋 autoDetect index 7→8 수정 (VBT26040Nxx)
- dps 기본값 1.771 → 1.968

UI:
- 도넛차트: Home버튼 삭제, Auto/Catheter count/History 삭제
- 3버튼 레이아웃 (Catheterize/Auto/Spot)
- 배터리 아이콘 (60+초록/30+주황/빨강) + 설정 톱니바퀴 분리
- SensorSelect/Consent 페이지 스킵
- 온보딩: VesiScan-Basic, NIRS 페이지 삭제
- MaxVolume: Step 표시 삭제, mL 오른쪽 배치
- BackHandler 전체 재정리 (자연스러운 뒤로가기)
- BLE 신호 기준 완화 (Good≥-80, Fair≥-90)

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

app/src/main/java/com/example/medilightv2android/MainActivity.kt

@@ -61,19 +61,25 @@ fun MedilightApp(modifier: Modifier = Modifier) {
     val context = LocalContext.current
     val appState = remember { AppState(context) }
 
-    // Android 뒤로가기 = 앱 내 뒤로가기
-    BackHandler(enabled = appState.currentScreen != AppScreen.HOME) {
+    // Android 뒤로가기 = 바로 전 화면
+    BackHandler(enabled = appState.currentScreen != AppScreen.HOME
+            && appState.currentScreen != AppScreen.PIN_ENTRY) {
         when (appState.currentScreen) {
-            AppScreen.SENSOR_SELECT -> appState.currentScreen = AppScreen.HOME
-            AppScreen.DEVICE_SCAN -> appState.currentScreen = AppScreen.SENSOR_SELECT
+            // 메인 플로우: Home → DeviceScan → Personalization → Placement → Monitoring
+            AppScreen.DEVICE_SCAN -> appState.currentScreen = AppScreen.HOME
             AppScreen.PIEZO_PERSONALIZATION -> appState.currentScreen = AppScreen.DEVICE_SCAN
-            AppScreen.PLACEMENT_GUIDE -> appState.goHome()
+            AppScreen.PLACEMENT_GUIDE -> appState.currentScreen = AppScreen.PIEZO_PERSONALIZATION
             AppScreen.PIEZO_MONITORING -> appState.currentScreen = AppScreen.PLACEMENT_GUIDE
-            AppScreen.VIVAMYO_MONITORING -> appState.goHome()
+            // 모니터링 하위 화면 → 모니터링
             AppScreen.MEASUREMENT_HISTORY -> appState.currentScreen = AppScreen.PIEZO_MONITORING
             AppScreen.VOIDING_DIARY -> appState.currentScreen = AppScreen.PIEZO_MONITORING
             AppScreen.REMINDER_SETTINGS -> appState.currentScreen = AppScreen.PIEZO_MONITORING
             AppScreen.URINE_CAMERA -> appState.currentScreen = AppScreen.PIEZO_MONITORING
+            // 기타
+            AppScreen.SENSOR_SELECT -> appState.currentScreen = AppScreen.HOME
+            AppScreen.VIVAMYO_MONITORING -> appState.currentScreen = AppScreen.HOME
+            AppScreen.NIRS_CALIBRATION -> appState.currentScreen = AppScreen.DEVICE_SCAN
+            // 온보딩/등록 (뒤로가기 막지 않되 Home으로)
             else -> appState.currentScreen = AppScreen.HOME
         }
     }