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/notification_service.dart

@@ -1,9 +1,11 @@
+import 'dart:io' show Platform, File;
 import 'dart:ui';
 
 import 'package:flutter_local_notifications/flutter_local_notifications.dart';
 import 'package:flutter/foundation.dart';
 
 import '../l10n/app_localizations.dart';
+import '../utils/platform_info.dart';
 
 class NotificationService {
   static final NotificationService _instance = NotificationService._internal();
@@ -75,6 +77,15 @@ class NotificationService {
       linux: linuxSettings,
     );
 
+    // On Linux, the notifications plugin opens a D-Bus session bus
+    // connection whose async subscription can throw an unhandled
+    // SocketException when the bus socket is missing (e.g. running as
+    // root or inside a container without a session bus).
+    if (PlatformInfo.isLinux && !_isDbusSessionAvailable()) {
+      debugPrint('Skipping notification init: D-Bus session bus unavailable');
+      return;
+    }
+
     try {
       await _notifications.initialize(
         settings: initSettings,
@@ -86,6 +97,16 @@ class NotificationService {
     }
   }
 
+  static bool _isDbusSessionAvailable() {
+    final addr = Platform.environment['DBUS_SESSION_BUS_ADDRESS'];
+    if (addr != null && addr.isNotEmpty) return true;
+    // Fallback: check the default socket for the current user.
+    final uid = Platform.environment['UID'] ??
+        Platform.environment['EUID'];
+    final path = '/run/user/${uid ?? '1000'}/bus';
+    return File(path).existsSync();
+  }
+
   Future<bool> _ensureInitialized() async {
     if (!_isInitialized) {
       await initialize();