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/services/usb_serial_service_native.dart

@@ -325,6 +325,10 @@ class UsbSerialService {
     // Native implementations do not use a synthetic chooser row.
   }
 
+  void setFallbackDeviceName(String label) {
+    // Native implementations use OS-provided device names.
+  }
+
   void updateConnectedLabel(String label) {
     final trimmed = label.trim();
     if (trimmed.isEmpty) {