Bump to Avalonia 12.0.3

Patch bump across the seven first-party Avalonia packages that share
$(AvaloniaVersion): Avalonia, Avalonia.Desktop, Avalonia.Themes.Fluent,
Avalonia.Themes.Simple, Avalonia.Fonts.Inter, Avalonia.Diagnostics,
Avalonia.Headless.NUnit. Third-party Avalonia-flavoured packages
(AvaloniaEdit, Xaml.Behaviors, Svg.Controls.Skia, Dock.Avalonia,
ProDataGrid) are on independent versions and untouched.

Assisted-by: Claude:claude-opus-4-7:Claude Code

Directory.Packages.props

@@ -7,7 +7,7 @@
     <RoslynVersion>5.7.0-1.26230.115</RoslynVersion>
     <NetPackageVersion>10.0.8</NetPackageVersion>
     <TomsToolboxVersion>2.24.0</TomsToolboxVersion>
-    <AvaloniaVersion>12.0.1</AvaloniaVersion>
+    <AvaloniaVersion>12.0.3</AvaloniaVersion>
     <DockVersion>12.0.0.1</DockVersion>
   </PropertyGroup>
   <ItemGroup>