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.

android/app/src/main/res/layout/activity_settings.xml

@@ -38,4 +38,17 @@
         android:layout_height="wrap_content"
         android:text="Firmware Flashen" />
 
+    <View
+        android:layout_width="0dp"
+        android:layout_height="0dp"
+        android:layout_weight="1" />
+
+    <TextView
+        android:id="@+id/buildNumber"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:textSize="12sp"
+        android:textColor="?android:attr/textColorSecondary"
+        android:gravity="center" />
+
 </LinearLayout>