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/app_debug_log_service.dart

@@ -52,7 +52,12 @@ class AppDebugLogService extends ChangeNotifier {
     String tag = 'App',
     AppDebugLogLevel level = AppDebugLogLevel.info,
   }) {
-    if (!_enabled) return;
+    if (!_enabled && !kDebugMode) return;
+    if (!_enabled) {
+      // In debug mode, still print to console but don't store entries.
+      debugPrint('[$tag] $message');
+      return;
+    }
 
     _entries.add(
       AppDebugLogEntry(