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/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()