Bump Avalonia to 12.0.4 for the DBus menu-exporter exception fix

On Linux desktops where the com.canonical.AppMenu.Registrar name is absent or
blinks in and out (e.g. KDE Plasma, where kappmenu owns it only transiently),
Avalonia's DBusMenuExporter fired an unguarded UnregisterWindowAsync on dispose.
The faulted fire-and-forget task went unobserved and was rethrown by the
finalizer as org.freedesktop.DBus.Error.ServiceUnknown ("The name is not
activatable"), surfacing through the global exception dialog. 12.0.4 observes
that task (AvaloniaUI/Avalonia#21344, fixing #17616); verified the ContinueWith
guard is present in the resolved Avalonia.FreeDesktop binary.

No lock-file churn: only the cross-platform decompiler libraries pin lock files
and none reference Avalonia. Full headless suite shows no new failures.

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>12.0.3</AvaloniaVersion>
+    <AvaloniaVersion>12.0.4</AvaloniaVersion>
     <DockVersion>12.0.0.1</DockVersion>
   </PropertyGroup>
   <ItemGroup>