feat: move mainwindow to core and add NavigationService

GSS2.UI.Core/MainWindow.axaml.cs

@@ -0,0 +1,16 @@
+using Avalonia.Controls;
+
+namespace GSS2.UI.Core;
+
+public partial class MainWindow : Window
+{
+    public MainWindow()
+    {
+        InitializeComponent();
+    }
+
+    public void SetControl(UserControl control)
+    {
+        MainContent.Content = control;
+    }
+}