feat: BLE 자동 reconnect + 배터리 폴링 5초 (keep-alive)

- BleManager: 배터리 폴링 30초→5초 (펌웨어 15초 타임아웃 방지, iOS 동일)
- BleManager: 자동 reconnect with 백오프 (3s→5s→10s, 최대 5회)
  - scheduleAutoReconnect() + attemptAutoReconnect()
  - GATT 에러 시에도 백오프 재시도
  - 수동 disconnect 시 자동 reconnect 취소
- PiezoMonitoringView: 끊김 시 즉시 파란 배너(RECONNECTING)
  - 성공 시 배너 자동 숨김 + 배터리 폴링 재시작
  - 5회 실패 시 빨간 배너 + 수동 Reconnect 버튼
  - 측정 중 끊김 → isMeasuring 리셋 + 자동측정 중지

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

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

@@ -56,6 +56,12 @@ class BleManager private constructor(private val context: Context) {
     private var batteryTimer: Runnable? = null
     private var connectionTimer: Runnable? = null
     private var isUserDisconnect = false
+    var lastConnectedAddress: String? = null
+        private set
+    private var reconnectAttempt = 0
+    private val maxReconnectAttempts = 5
+    private val reconnectDelays = longArrayOf(3000, 5000, 10000, 10000, 10000)
+    private var reconnectTimer: Runnable? = null
 
     val discoveredDevices = mutableStateListOf<BleDevice>()
     val isScanning = mutableStateOf(false)
@@ -212,6 +218,7 @@ class BleManager private constructor(private val context: Context) {
 
     fun disconnect() {
         isUserDisconnect = true
+        cancelAutoReconnect()
         connectionTimer?.let { handler.removeCallbacks(it) }
         connectionTimer = null
         bluetoothGatt?.disconnect()
@@ -321,16 +328,15 @@ class BleManager private constructor(private val context: Context) {
 
     fun startBatteryPolling() {
         batteryTimer?.let { handler.removeCallbacks(it) }
-        // Query immediately, then every 30s
         sendBatteryQuery()
         val poll = object : Runnable {
             override fun run() {
                 sendBatteryQuery()
-                handler.postDelayed(this, 30000)
+                handler.postDelayed(this, 5000)
             }
         }
         batteryTimer = poll
-        handler.postDelayed(poll, 30000)
+        handler.postDelayed(poll, 5000)
     }
 
     fun stopBatteryPolling() {
@@ -338,6 +344,57 @@ class BleManager private constructor(private val context: Context) {
         batteryTimer = null
     }
 
+    // ── Auto Reconnect ──
+
+    private fun scheduleAutoReconnect() {
+        if (reconnectAttempt >= maxReconnectAttempts) {
+            logw { "Max reconnect attempts ($maxReconnectAttempts) reached" }
+            isReconnecting.value = false
+            onReconnectionFailed?.invoke()
+            return
+        }
+
+        val delay = reconnectDelays[reconnectAttempt.coerceAtMost(reconnectDelays.size - 1)]
+        logd { "Auto-reconnect attempt ${reconnectAttempt + 1}/$maxReconnectAttempts in ${delay}ms" }
+        isReconnecting.value = true
+
+        cancelReconnectTimer()
+        val timer = Runnable { attemptAutoReconnect() }
+        reconnectTimer = timer
+        handler.postDelayed(timer, delay)
+    }
+
+    private fun attemptAutoReconnect() {
+        val address = lastConnectedAddress ?: return
+        if (bluetoothAdapter?.isEnabled != true) {
+            onReconnectionFailed?.invoke()
+            isReconnecting.value = false
+            return
+        }
+
+        reconnectAttempt++
+        logd { "Attempting auto-reconnect #$reconnectAttempt to $address" }
+
+        bluetoothGatt?.let { gatt ->
+            try { gatt.close() } catch (_: Exception) {}
+            bluetoothGatt = null
+        }
+
+        val device = bluetoothAdapter.getRemoteDevice(address)
+        bluetoothGatt = device.connectGatt(context, true, gattCallback, BluetoothDevice.TRANSPORT_LE)
+    }
+
+    private fun cancelReconnectTimer() {
+        reconnectTimer?.let { handler.removeCallbacks(it) }
+        reconnectTimer = null
+    }
+
+    fun cancelAutoReconnect() {
+        cancelReconnectTimer()
+        reconnectAttempt = 0
+        isReconnecting.value = false
+    }
+
     private val gattCallback = object : BluetoothGattCallback() {
         override fun onConnectionStateChange(gatt: BluetoothGatt, status: Int, newState: Int) {
             logd { "onConnectionStateChange: status=$status newState=$newState" }
@@ -347,20 +404,24 @@ class BleManager private constructor(private val context: Context) {
                 connectionTimer = null
 
                 if (status != BluetoothGatt.GATT_SUCCESS && newState != BluetoothProfile.STATE_CONNECTED) {
-                    // Connection failed with error
                     loge { "GATT connection failed with status: $status" }
                     bluetoothGatt = null
                     txCharacteristic = null
                     rxCharacteristic = null
                     isConnected.value = false
                     isServiceReady.value = false
-                    isReconnecting.value = false
-                    connectionError.value = "Connection failed (status $status). Try again."
 
                     val idx = discoveredDevices.indexOfFirst { it.address == gatt.device.address }
                     if (idx >= 0) discoveredDevices[idx] = discoveredDevices[idx].copy(isConnected = false, isConnecting = false)
 
                     gatt.close()
+
+                    if (reconnectAttempt > 0 && lastConnectedAddress != null) {
+                        scheduleAutoReconnect()
+                    } else {
+                        isReconnecting.value = false
+                        connectionError.value = "Connection failed (status $status). Try again."
+                    }
                     return@post
                 }
 
@@ -378,7 +439,10 @@ class BleManager private constructor(private val context: Context) {
                         com.example.medilightv2android.managers.PiezoHW.autoDetectPreset(connectedDeviceName.value)
                         logd { "Device preset: ${com.example.medilightv2android.managers.PiezoHW.presetName} (from '${connectedDeviceName.value}')" }
 
-                        // Save last connected device address
+                        lastConnectedAddress = gatt.device.address
+                        reconnectAttempt = 0
+                        cancelReconnectTimer()
+
                         try {
                             val prefs = context.getSharedPreferences("medilight_prefs", Context.MODE_PRIVATE)
                             prefs.edit().putString("last_device_address", gatt.device.address).apply()
@@ -403,8 +467,9 @@ class BleManager private constructor(private val context: Context) {
 
                         onConnectionStateChanged?.invoke(false)
 
-                        if (!isUserDisconnect) {
+                        if (!isUserDisconnect && lastConnectedAddress != null) {
                             onUnexpectedDisconnect?.invoke()
+                            scheduleAutoReconnect()
                         }
                         isUserDisconnect = false
                         gatt.close()