feat: UI 일괄 수정 (5,6,7,8번)

5. Onboarding 간소화: 4단계→1단계 (Max Volume만)
   Step 1(자세)/2(위치)/4(최적화) 제거, Continue 바로 완료

6. 자동 reconnect: 앱 시작 시 마지막 기기에 자동 연결
   SharedPreferences에 address+name 저장, HomeView에서 autoConnectLastDevice()

7. Demo 모드: DeviceScanView에서 Demo 버튼 제거

8. Android 뒤로가기: BackHandler로 화면별 네비게이션 매핑
   Placement→Home, Monitoring→Placement, History/Diary→Monitoring 등

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

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

@@ -2,6 +2,7 @@ package com.example.medilightv2android
 
 import android.os.Bundle
 import androidx.activity.ComponentActivity
+import androidx.activity.compose.BackHandler
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
 import androidx.compose.animation.Crossfade
@@ -60,6 +61,23 @@ 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) {
+        when (appState.currentScreen) {
+            AppScreen.SENSOR_SELECT -> appState.currentScreen = AppScreen.HOME
+            AppScreen.DEVICE_SCAN -> appState.currentScreen = AppScreen.SENSOR_SELECT
+            AppScreen.PIEZO_PERSONALIZATION -> appState.currentScreen = AppScreen.DEVICE_SCAN
+            AppScreen.PLACEMENT_GUIDE -> appState.goHome()
+            AppScreen.PIEZO_MONITORING -> appState.currentScreen = AppScreen.PLACEMENT_GUIDE
+            AppScreen.VIVAMYO_MONITORING -> appState.goHome()
+            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
+            else -> appState.currentScreen = AppScreen.HOME
+        }
+    }
+
     Crossfade(
         targetState = appState.currentScreen,
         animationSpec = tween(300),

app/src/main/java/com/example/medilightv2android/ble/BleManager.kt

@@ -221,6 +221,17 @@ class BleManager private constructor(private val context: Context) {
         handler.postDelayed(timeout, 10000)
     }
 
+    /** 저장된 마지막 기기에 자동 연결 시도 */
+    fun autoConnectLastDevice() {
+        if (isConnected.value) return
+        val prefs = context.getSharedPreferences("medilight_prefs", Context.MODE_PRIVATE)
+        val address = prefs.getString("last_device_address", null) ?: return
+        val name = prefs.getString("last_device_name", null) ?: return
+        debugLogger.info("AUTO_CONNECT attempting $name ($address)")
+        lastConnectedAddress = address
+        scheduleAutoReconnect()
+    }
+
     fun disconnect() {
         isUserDisconnect = true
         cancelAutoReconnect()
@@ -614,7 +625,10 @@ class BleManager private constructor(private val context: Context) {
 
                         try {
                             val prefs = context.getSharedPreferences("medilight_prefs", Context.MODE_PRIVATE)
-                            prefs.edit().putString("last_device_address", gatt.device.address).apply()
+                            prefs.edit()
+                                .putString("last_device_address", gatt.device.address)
+                                .putString("last_device_name", connectedDeviceName.value)
+                                .apply()
                         } catch (_: Exception) {}
 
                         gatt.discoverServices()

app/src/main/java/com/example/medilightv2android/ui/views/connection/DeviceScanView.kt

@@ -249,16 +249,6 @@ fun DeviceScanView(appState: AppState) {
             }
         }
 
-        // Demo mode button
-        TextButton(
-            onClick = { appState.enterDemoMode() },
-            modifier = Modifier
-                .fillMaxWidth()
-                .padding(horizontal = 24.dp)
-        ) {
-            Text("Demo Mode (No Device)", fontSize = 14.sp, color = MlSecondaryText)
-        }
-
         // Scan button
         Button(
             onClick = {

app/src/main/java/com/example/medilightv2android/ui/views/home/HomeView.kt

@@ -8,6 +8,9 @@ import androidx.compose.material3.Button
 import androidx.compose.material3.ButtonDefaults
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.ui.platform.LocalContext
+import com.example.medilightv2android.ble.BleManager
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
@@ -22,6 +25,12 @@ import com.example.medilightv2android.ui.theme.*
 
 @Composable
 fun HomeView(appState: AppState) {
+    val context = LocalContext.current
+    // 앱 시작 시 마지막 기기에 자동 연결 시도
+    LaunchedEffect(Unit) {
+        BleManager.getInstance(context).autoConnectLastDevice()
+    }
+
     Column(
         modifier = Modifier
             .fillMaxSize()

app/src/main/java/com/example/medilightv2android/ui/views/personalization/PiezoPersonalizationView.kt

@@ -37,7 +37,7 @@ fun PiezoPersonalizationView(appState: AppState) {
     val handler = remember { Handler(Looper.getMainLooper()) }
     var currentStep by remember { mutableIntStateOf(1) }
     var optimizationComplete by remember { mutableStateOf(false) }
-    val totalSteps = 4
+    val totalSteps = 1
 
     fun startOptimization() {
         bleManager.onPiezoDataReceived = {
@@ -88,43 +88,15 @@ fun PiezoPersonalizationView(appState: AppState) {
 
         Spacer(modifier = Modifier.height(24.dp))
 
-        // Step content
+        // Max Volume만 표시
         when (currentStep) {
-            1 -> PostureStep(appState)
-            2 -> SensorPositionStep(appState)
-            3 -> MaxVolumeStep(appState)
-            4 -> {
-                LaunchedEffect(Unit) { startOptimization() }
-                OptimizationStep(optimizationComplete)
-            }
+            1 -> MaxVolumeStep(appState)
         }
 
         Spacer(modifier = Modifier.weight(1f))
 
         // Navigation button
-        if (currentStep < 4) {
-            Button(
-                onClick = { currentStep++ },
-                modifier = Modifier
-                    .fillMaxWidth()
-                    .padding(horizontal = 32.dp)
-                    .padding(bottom = 32.dp)
-                    .height(56.dp),
-                shape = RoundedCornerShape(16.dp),
-                colors = ButtonDefaults.buttonColors(containerColor = Color.Transparent),
-                contentPadding = PaddingValues()
-            ) {
-                Box(
-                    modifier = Modifier.fillMaxSize().background(
-                        Brush.horizontalGradient(listOf(MlTeal, MlPrimary)),
-                        RoundedCornerShape(16.dp)
-                    ),
-                    contentAlignment = Alignment.Center
-                ) {
-                    Text("Next", fontSize = 18.sp, fontWeight = FontWeight.SemiBold, color = Color.White)
-                }
-            }
-        } else if (optimizationComplete) {
+        if (currentStep <= totalSteps) {
             Button(
                 onClick = { appState.personalizationComplete() },
                 modifier = Modifier