Добавлена кнопка и окно "О приложении"

Добавлена кнопка "О приложении" в окне настроек (SettingsWindow.cs), а также окно "О приложении" (AboutWindow.cs)

src/KeyKeeper/AboutWindow.cs

@@ -0,0 +1,50 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Media;
+
+namespace KeyKeeper.Views;
+public class AboutWindow : Window
+{
+    public AboutWindow()
+    {
+        this.Title = "О приложении";
+        this.Width = 600;
+        this.Height = 400;
+
+        var AboutKeyKeeper = new TextBlock
+        {
+            Text = "About Keykeeper",
+            HorizontalAlignment = HorizontalAlignment.Left,
+            FontSize = 50,
+            TextAlignment = TextAlignment.Left
+        };
+
+        var AboutText = new TextBlock
+        {
+            Text = "KeyKeeper is a personal password and key manager\nwhere you can save passwords and other login\ninformation, configure one-time code generation,\nand create encryption keys for personal use",
+            HorizontalAlignment = HorizontalAlignment.Left,
+            FontSize = 16,
+            TextAlignment = TextAlignment.Left,
+            Margin = new Thickness(0, 20, 0, 0)
+        };
+
+        var mainGrid = new Grid
+        {
+            VerticalAlignment = VerticalAlignment.Center,
+            HorizontalAlignment = HorizontalAlignment.Center
+        };
+
+        var innerStack = new StackPanel
+        {
+            Width = 400
+        };
+
+        innerStack.Children.Add(AboutKeyKeeper);
+        innerStack.Children.Add(AboutText);
+
+        mainGrid.Children.Add(innerStack);
+
+        this.Content = mainGrid;
+    }
+}

src/KeyKeeper/SettingsWindow.cs

@@ -5,18 +5,27 @@ using Avalonia.Media;
 namespace KeyKeeper.Views;
 public class SettingsWindow : Window
 {
+    private async void OpenAbout()
+    {
+        var AboutWindow = new AboutWindow();
+        await AboutWindow.ShowDialog(this);
+    }
+
     public SettingsWindow()
     {
         this.Title = "Настройки";
         this.Width = 400;
         this.Height = 300;
-        var textBlock = new TextBlock
+        var AboutButton = new Button
         {
-            Text = "Окно настроек",
+            Content = "О приложении",
             HorizontalAlignment = Avalonia.Layout.HorizontalAlignment.Center,
             VerticalAlignment = Avalonia.Layout.VerticalAlignment.Center,
             FontSize = 16
         };
-        this.Content = textBlock;
+
+        AboutButton.Click += (sender, e) => OpenAbout();
+
+        this.Content = AboutButton;
     }
 }