fix: detecteer ontbrekende OTA characteristic en toon USB-flash melding

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

@@ -197,6 +197,10 @@ class BleRelayManager(
         val gatt = bluetoothGatt ?: return false
         return gatt.setCharacteristicNotification(char, true)
     }
+
+    fun hasOtaCharacteristic(): Boolean {
+        return otaCharacteristic != null
+    }
     fun updateKeys() {
         loadState()
     }

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

@@ -215,6 +215,10 @@ class SettingsActivity : AppCompatActivity() {
             Toast.makeText(this, "Niet verbonden met SylDa_Server", Toast.LENGTH_SHORT).show()
             return
         }
+        if (!manager.hasOtaCharacteristic()) {
+            Toast.makeText(this, getString(R.string.ota_needs_usb_first), Toast.LENGTH_LONG).show()
+            return
+        }
         otaManager = OtaManager(this, manager)
 
         if (otaManager?.loadFirmware() != true) {

android/app/src/main/res/values/strings.xml

@@ -112,4 +112,5 @@
     <string name="ota_complete">Update voltooid! ESP herstart...</string>
     <string name="ota_failed">OTA update mislukt: %s</string>
     <string name="ota_load_failed">Kan firmware bestand niet laden</string>
+    <string name="ota_needs_usb_first">Module heeft nog geen OTA-ondersteuning — eerst flashen via USB</string>
 </resources>