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/services/usb_serial_service_native.dart

@@ -280,6 +280,11 @@ class UsbSerialService {
   Future<void> disconnect() async {
     if (_status == UsbSerialStatus.disconnected) return;
 
+    final portLabel = _connectedPortLabel ?? _connectedPortKey;
+    _debugLogService?.info(
+      'USB disconnect starting port=${portLabel ?? 'unknown'}',
+      tag: 'USB Serial',
+    );
     _status = UsbSerialStatus.disconnecting;
     _connectedPortKey = null;
     _connectedPortLabel = null;
@@ -319,6 +324,10 @@ class UsbSerialService {
       _dataSubscription = null;
     }
     _status = UsbSerialStatus.disconnected;
+    _debugLogService?.info(
+      'USB disconnect complete port=${portLabel ?? 'unknown'}',
+      tag: 'USB Serial',
+    );
   }
 
   void setRequestPortLabel(String label) {