feat(ui): add Quit to File menu; hide Wallet item when no wallet is open

- File → Esci (separator + Quit at the bottom) calls IClassicDesktopStyleApplicationLifetime.Shutdown()
- Wallet menu item and mobile button now use IsVisible instead of IsEnabled:
  they disappear entirely on the wizard/setup screen and reappear once a
  wallet is open, avoiding a confusing greyed-out entry

src/App/ViewModels/MainWindowViewModel.Settings.cs

@@ -69,6 +69,12 @@ public partial class MainWindowViewModel
     [RelayCommand]
     private void OpenHelp() => IsHelpOpen = true;
 
+    [RelayCommand]
+    private static void QuitApp() =>
+        (Avalonia.Application.Current?.ApplicationLifetime
+            as Avalonia.Controls.ApplicationLifetimes.IClassicDesktopStyleApplicationLifetime)
+            ?.Shutdown();
+
     [RelayCommand]
     private void CloseHelp() { IsHelpOpen = false; ResetDonate(); }
 }