Add project restore files and NuGet cache for GsaViewer

- Created project.nuget.cache to store NuGet package cache information.
- Added project.packagespec.json to define project restore settings and dependencies.
- Included rider.project.restore.info for Rider IDE integration.

GsaEditor/Views/MainWindow.axaml.cs

@@ -0,0 +1,52 @@
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using GsaEditor.ViewModels;
+
+namespace GsaEditor.Views;
+
+public partial class MainWindow : Window
+{
+    private bool _forceClose;
+
+    public MainWindow()
+    {
+        InitializeComponent();
+    }
+
+    protected override void OnLoaded(RoutedEventArgs e)
+    {
+        base.OnLoaded(e);
+
+        if (DataContext is MainWindowViewModel vm)
+        {
+            vm.SetWindow(this);
+        }
+    }
+
+    private void TreeView_SelectionChanged(object? sender, SelectionChangedEventArgs e)
+    {
+        if (DataContext is MainWindowViewModel vm && sender is TreeView tree)
+        {
+            vm.SelectedNode = tree.SelectedItem as EntryTreeNodeViewModel;
+        }
+    }
+
+    protected override async void OnClosing(WindowClosingEventArgs e)
+    {
+        base.OnClosing(e);
+
+        if (_forceClose) return;
+
+        if (DataContext is MainWindowViewModel vm && vm.IsDirty)
+        {
+            e.Cancel = true;
+            var confirmed = await Helpers.Dialogs.ConfirmAsync(this, "Unsaved Changes",
+                "You have unsaved changes. Close without saving?");
+            if (confirmed)
+            {
+                _forceClose = true;
+                Close();
+            }
+        }
+    }
+}