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/l10n/app_localizations_bg.dart

@@ -175,7 +175,25 @@ class AppLocalizationsBg extends AppLocalizations {
 
   @override
   String get usbErrorConnectTimedOut =>
-      'Изчаква се, но устройството не отговаря в рамките на зададения време.';
+      'Връзката прекъсна. Уверете се, че устройството има софтуер за USB връзка.';
+
+  @override
+  String get usbFallbackDeviceName =>
+      'Устройство за четене на уеб серийни данни';
+
+  @override
+  String get usbStatus_notConnected => 'Изберете USB устройство';
+
+  @override
+  String get usbStatus_connecting => 'Свързване към USB устройство...';
+
+  @override
+  String get usbStatus_searching => 'Търсене на USB устройства...';
+
+  @override
+  String usbConnectionFailed(String error) {
+    return 'Неуспешно свързване през USB: $error';
+  }
 
   @override
   String get scanner_scanning => 'Сканиране за устройства...';