Use named Runnable for scheduled BLE scans and cancel on pause

Replace anonymous lambdas in handler.postDelayed with a shared
scanRunnable so that handler.removeCallbacks(scanRunnable) in onPause
cancels any pending scan. Prevents duplicate or orphaned scan starts
during rapid activity transitions.

Reviewed in codereview.md (recommendation 2.2).

android/app/src/main/java/com/eddy/atomrelay/MainActivity.kt

@@ -49,6 +49,7 @@ class MainActivity : AppCompatActivity() {
     private var isScanning = false
     private var scanTimedOut = false
     private val handler = Handler(Looper.getMainLooper())
+    private val scanRunnable = Runnable { startScan() }
 
     private var seq = 1
     private var hmacKey: ByteArray? = null
@@ -89,7 +90,7 @@ class MainActivity : AppCompatActivity() {
                 bluetoothGatt = null
                 relayCharacteristic = null
                 seq = 1
-                handler.postDelayed({ startScan() }, 2000)
+                handler.postDelayed(scanRunnable, 2000)
             }
         }
 
@@ -165,7 +166,7 @@ class MainActivity : AppCompatActivity() {
             relayCharacteristic = null
             seq = 1
             loadHmacKey()
-            handler.postDelayed({ startScan() }, 500)
+            handler.postDelayed(scanRunnable, 500)
         }
     }
 
@@ -175,12 +176,13 @@ class MainActivity : AppCompatActivity() {
         if (hmacKey == null) {
             statusText.text = "Geen HMAC sleutel - open Instellingen"
         } else if (!isScanning && bluetoothGatt == null) {
-            handler.postDelayed({ startScan() }, 300)
+            handler.postDelayed(scanRunnable, 300)
         }
     }
 
     override fun onPause() {
         super.onPause()
+        handler.removeCallbacks(scanRunnable)
         stopScan()
     }