Replace BLE encryption with HMAC-only auth, add buildnr, fix post-flash flow

BLE bonding/encryption removed — HMAC-SHA256 on application level provides
authentication + replay protection. Eliminates bonding state bugs, write
queue issues, and GATT_INSUFFICIENT_AUTHENTICATION complexity.

Firmware:
- Remove BLESecurity, setRespEncryptionKey, ESP_GATT_PERM_WRITE_ENCRYPTED
- Characteristic permission: ESP_GATT_PERM_WRITE (no encryption required)
- Characteristic property: PROPERTY_WRITE_NR (Write Without Response)
  Enables direct ON/OFF sequences without ACK wait time

Android:
- Remove bondReceiver, createBond, bonding state handling
- Remove write queue / pendingRelayState / writeInProgress
- Use WRITE_TYPE_NO_RESPONSE for minimal latency
- Post-flash dialog auto-dismisses on USB detach (currentAction = NONE)
- Separate progressDialog (flash progress) from dialog (post-flash info)
- Detach during active action now calls cleanupAction()

Build number:
- BuildConfig.BUILD_NUMBER in build.gradle as yyyyMMddHHmmss timestamp
- buildFeatures.buildConfig enabled
- Displayed in SettingsActivity bottom (12sp, secondary color)

Gradle:
- Wrapper upgraded 8.5 → 8.14
- minSdk 24 → 26 (Android 8+)

README updated to reflect HMAC-only security model.

src/main.cpp

@@ -3,7 +3,7 @@
 #include <BLEServer.h>
 #include <BLEUtils.h>
 #include <BLE2902.h>
-#include <BLESecurity.h>
+
 #include <mbedtls/md.h>
 #include <Preferences.h>
 
@@ -150,20 +150,15 @@ void setup() {
 
     BLEDevice::init("AtomS3R-Relay");
 
-    BLESecurity *pSecurity = new BLESecurity();
-    pSecurity->setAuthenticationMode(ESP_LE_AUTH_REQ_SC_BOND);
-    pSecurity->setCapability(ESP_IO_CAP_NONE);
-    pSecurity->setInitEncryptionKey(ESP_BLE_ENC_KEY_MASK | ESP_BLE_ID_KEY_MASK);
-
     BLEServer *pServer = BLEDevice::createServer();
     pServer->setCallbacks(new MyServerCallbacks());
 
     BLEService *pService = pServer->createService(SERVICE_UUID);
     pCharacteristic = pService->createCharacteristic(
         CHARACTERISTIC_UUID,
-        BLECharacteristic::PROPERTY_WRITE | BLECharacteristic::PROPERTY_NOTIFY
+        BLECharacteristic::PROPERTY_WRITE_NR | BLECharacteristic::PROPERTY_NOTIFY
     );
-    pCharacteristic->setAccessPermissions(ESP_GATT_PERM_WRITE_ENCRYPTED);
+    pCharacteristic->setAccessPermissions(ESP_GATT_PERM_WRITE);
     pCharacteristic->addDescriptor(new BLE2902());
     pCharacteristic->setCallbacks(new MyCharacteristicCallbacks());
     pCharacteristic->setValue("0");