Add onCharacteristicWrite callback for BLE write feedback

Implement onCharacteristicWrite in gattCallback to get confirmation
that packets are submitted to the local BLE stack. Logs success/failure
and shows 'Communicatiefout' on write failure. seq++ stays in
sendRelayCommand to ensure unique sequence numbers for rapid ON/OFF.

Reviewed in codereview.md (recommendation #5).

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

@@ -118,6 +118,16 @@ class MainActivity : AppCompatActivity() {
                 runOnUiThread { statusText.text = "Service not found" }
             }
         }
+
+        @SuppressLint("MissingPermission")
+        override fun onCharacteristicWrite(gatt: BluetoothGatt, characteristic: BluetoothGattCharacteristic, status: Int) {
+            if (status == BluetoothGatt.GATT_SUCCESS) {
+                Log.d(TAG, "Write confirmed by local BLE stack (status=$status)")
+            } else {
+                Log.w(TAG, "Write failed (status=$status)")
+                runOnUiThread { statusText.text = "Communicatiefout" }
+            }
+        }
     }
 
     override fun onCreate(savedInstanceState: Bundle?) {