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/build.gradle

@@ -9,10 +9,15 @@ android {
 
     defaultConfig {
         applicationId "com.eddy.atomrelay"
-        minSdk 24
+        minSdk 26
         targetSdk 34
         versionCode 1
         versionName "1.0"
+        buildConfigField "String", "BUILD_NUMBER", "\"${new java.text.SimpleDateFormat("yyyyMMddHHmmss").format(new Date())}\""
+    }
+
+    buildFeatures {
+        buildConfig true
     }
 
     buildTypes {