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(

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();

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) {

lib/services/usb_serial_service_web.dart

@@ -32,6 +32,7 @@ class UsbSerialService {
   String? _connectedPortName;
   String? _connectedPortKey;
   String _requestPortLabel = 'Choose USB Device';
+  String _fallbackDeviceName = 'Web Serial Device';
   AppDebugLogService? _debugLogService;
 
   UsbSerialStatus get status => _status;
@@ -77,11 +78,19 @@ class UsbSerialService {
 
     try {
       final requestedPortName = normalizeUsbPortName(portName);
+      _debugLogService?.info(
+        'Web connect: requested=$requestedPortName baud=$baudRate',
+        tag: 'USB Serial',
+      );
       final selectedPortKey = requestedPortName.startsWith('web:port:')
           ? requestedPortName
           : null;
       _port = _authorizedPortsByKey[requestedPortName];
       final authorizedPorts = await _getAuthorizedPorts();
+      _debugLogService?.info(
+        'Web connect: ${authorizedPorts.length} authorized port(s), cached=${_port != null}',
+        tag: 'USB Serial',
+      );
       _port ??= _selectPort(authorizedPorts, requestedPortName);
 
       _port ??= await _requestPort();
@@ -89,6 +98,10 @@ class UsbSerialService {
         throw StateError('No USB serial device selected');
       }
 
+      _debugLogService?.info(
+        'Web connect: opening port at $baudRate baud…',
+        tag: 'USB Serial',
+      );
       await _openPort(_port!, baudRate);
       _connectedPortKey = _cachePort(_port!, preferredKey: selectedPortKey);
       _connectedPortName = _displayLabelForPort(
@@ -105,6 +118,10 @@ class UsbSerialService {
         tag: 'USB Serial',
       );
     } catch (error) {
+      _debugLogService?.error(
+        'Web connect failed: $error',
+        tag: 'USB Serial',
+      );
       await _cleanupFailedConnect();
       _status = UsbSerialStatus.disconnected;
       _connectedPortName = null;
@@ -194,6 +211,14 @@ class UsbSerialService {
     _requestPortLabel = trimmed;
   }
 
+  void setFallbackDeviceName(String label) {
+    final trimmed = label.trim();
+    if (trimmed.isEmpty) {
+      return;
+    }
+    _fallbackDeviceName = trimmed;
+  }
+
   void setDebugLogService(AppDebugLogService? service) {
     _debugLogService = service;
   }
@@ -403,6 +428,7 @@ class UsbSerialService {
       vendorId: hasVendor ? vendorId : null,
       productId: hasProduct ? productId : null,
       requestPortLabel: _requestPortLabel,
+      fallbackDeviceName: _fallbackDeviceName,
       knownUsbNames: _knownUsbNames,
     );
   }