Refine USB transport flow

- replace Android USB dependency with app-owned USB host implementation\n- restore BLE-first scanner flow with USB secondary action\n- tighten Web Serial key handling and disconnect logging\n\nTODO (follow-up):\n- review non-English localization copy for tone and consistency\n- trim remaining unused/awkward localization strings introduced during USB UI changes

lib/utils/dialog_utils.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import '../connector/meshcore_connector.dart';
 import '../l10n/l10n.dart';
+import 'app_logger.dart';
 
 /// Shows a confirmation dialog before disconnecting from the device.
 /// Returns true if user confirmed and disconnect completed, false otherwise.
@@ -28,6 +29,7 @@ Future<bool> showDisconnectDialog(
   );
 
   if (confirmed == true) {
+    appLogger.info('Disconnect confirmed from popup', tag: 'Connection');
     await connector.disconnect();
     return true;
   }