fix: NimBLE 128-bit UUID scan response + Android key/UUID loading

- Firmware: add explicit setScanResponseData with setCompleteServices for
  128-bit service UUID so Android ScanFilter finds the device
- Android: hexStringToBytes accepts any even-length hex (fixes UUID=32chars
  failing the hardcoded 64-char check)
- Android: onActivityResult matches SettingsActivity.RESULT_SCAN instead of
  Activity.RESULT_OK

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

@@ -213,7 +213,7 @@ class MainActivity : AppCompatActivity() {
     @Deprecated("Deprecated in Java")
     override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
         super.onActivityResult(requestCode, resultCode, data)
-        if (requestCode == REQUEST_SETTINGS && resultCode == Activity.RESULT_OK) {
+        if (requestCode == REQUEST_SETTINGS && resultCode == SettingsActivity.RESULT_SCAN) {
             stopScan()
             bluetoothGatt?.close()
             bluetoothGatt = null
@@ -260,7 +260,7 @@ class MainActivity : AppCompatActivity() {
 
     private fun hexStringToBytes(hex: String): ByteArray? {
         val cleaned = hex.uppercase().filter { it in "0123456789ABCDEF" }
-        if (cleaned.length != 64) return null
+        if (cleaned.isEmpty() || cleaned.length % 2 != 0) return null
         return cleaned.chunked(2).map { it.toInt(16).toByte() }.toByteArray()
     }