Add about window

Views/AboutWindow.axaml

@@ -0,0 +1,16 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+        x:Class="ReceiptPDFBuilder.AboutWindow"
+        Title="About Receipt PDF Builder"
+        Width="300"
+        Height="300"
+        MaxHeight="400"
+        MaxWidth="400"
+        MinWidth="300"
+        MinHeight="300">
+  <TextBlock Text="Receipt PDF Builder was built by MB for A in 2026. Thanks for using!"
+             TextWrapping="Wrap"/>
+</Window>

Views/AboutWindow.axaml.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace ReceiptPDFBuilder;
+
+public partial class AboutWindow : Window
+{
+    public AboutWindow()
+    {
+        InitializeComponent();
+    }
+}