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/main.dart

@@ -8,7 +8,7 @@ import 'screens/chrome_required_screen.dart';
 import 'utils/platform_info.dart';
 
 import 'connector/meshcore_connector.dart';
-import 'screens/connection_choice_screen.dart';
+import 'screens/scanner_screen.dart';
 import 'services/storage_service.dart';
 import 'services/message_retry_service.dart';
 import 'services/path_history_service.dart';
@@ -192,7 +192,7 @@ class MeshCoreApp extends StatelessWidget {
             },
             home: (PlatformInfo.isWeb && !PlatformInfo.isChrome)
                 ? const ChromeRequiredScreen()
-                : const ConnectionChoiceScreen(),
+                : const ScannerScreen(),
           );
         },
       ),