Refactor USB screen, add debug logging, fix UI issues

- Rewrite UsbScreen to mirror ScannerScreen patterns (status bar,
  tap-to-connect port list, bottom FABs, SnackBar errors)
- Extract MeshCoreUsbManager from MeshCoreConnector for cleaner
  USB transport ownership
- Add debug logging throughout USB connection flow (connector,
  manager, web/native services)
- Print debug logs to console in debug mode even when app debug
  log setting is disabled
- Localize remaining hardcoded strings (Web Serial Device fallback
  label, USB status bar keys, companion firmware timeout hint)
- Fix Swedish misspelling in translations (stöderliga → stödda)
- Guard Linux notification init against missing D-Bus session bus
- Fix SNRIndicator hit-test error by adding minimum size constraints
- Update USB flow tests for new UI patterns

lib/utils/usb_port_labels.dart

@@ -31,6 +31,7 @@ String describeWebUsbPort({
   required int? vendorId,
   required int? productId,
   String requestPortLabel = 'Choose USB Device',
+  String fallbackDeviceName = 'Web Serial Device',
   Map<String, String> knownUsbNames = const <String, String>{},
 }) {
   if (vendorId == null && productId == null) {
@@ -43,7 +44,7 @@ String describeWebUsbPort({
       ? knownUsbNames['${vendorHex.toLowerCase()}:${productHex.toLowerCase()}']
       : null;
 
-  final parts = <String>[knownName ?? 'Web Serial Device'];
+  final parts = <String>[knownName ?? fallbackDeviceName];
   if (vendorHex != null) {
     parts.add('VID:$vendorHex');
   }