Add device UUID prefix to BLE scan response and filter in scan callback

ESP32 server: include 4-byte device UUID prefix as manufacturer data
(company 0xFFFF) in BLE scan response, allowing pre-connection filtering.

Android: check manufacturer data in ScanCallback.onScanResult before
connecting. Skips devices with non-matching UUID prefix, preventing
wasted connections to wrong SylDa devices. Backward compatible with
old firmware (proceeds if no manufacturer data present).

android/app/src/main/java/nl/eddydevink/sylda/BleRelayManager.kt

@@ -89,6 +89,19 @@ class BleRelayManager(
                 Log.d(TAG, "Skipping already-tried device $address")
                 return
             }
+            val scanRecord = result.scanRecord
+            val storedUuid = deviceUuid
+            if (scanRecord != null && storedUuid != null) {
+                val mfgData = scanRecord.getManufacturerSpecificData(0xFFFF)
+                if (mfgData != null && mfgData.size >= 4) {
+                    val advPrefix = mfgData.copyOf(4)
+                    val storedPrefix = storedUuid.copyOf(4)
+                    if (!advPrefix.contentEquals(storedPrefix)) {
+                        Log.d(TAG, "Skipping non-matching UUID prefix ($address)")
+                        return
+                    }
+                }
+            }
             stopScan()
             connectToDevice(device)
         }