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

src/main.cpp

@@ -1,5 +1,6 @@
 #include <Arduino.h>
 #include <NimBLEDevice.h>
+#include <NimBLEAdvertising.h>
 
 #include <mbedtls/md.h>
 #include <Preferences.h>
@@ -202,13 +203,19 @@ void setup() {
     pUuidChar->setValue(deviceUuid, ESP_UUID_LEN);
 
     NimBLEAdvertising *pAdvertising = NimBLEDevice::getAdvertising();
+    pAdvertising->setName("SylDa_Server");
     pAdvertising->addServiceUUID(SERVICE_UUID);
+
+    NimBLEAdvertisementData scanResp;
+    scanResp.setCompleteServices(NimBLEUUID(SERVICE_UUID));
+    pAdvertising->setScanResponseData(scanResp);
     pAdvertising->enableScanResponse(true);
+
     pAdvertising->setMinInterval(32);
     pAdvertising->setMaxInterval(64);
-    NimBLEDevice::startAdvertising();
-
-    Serial.println("BLE: advertising started as 'SylDa_Server'");
+    bool advOk = pAdvertising->start();
+    Serial.printf("BLE: advertising started as 'SylDa_Server' (start=%d, isAdv=%d)\n",
+                  advOk, pAdvertising->isAdvertising());
 
     if (keyProvisioned) {
         Serial.print("HMAC Key: ");