Fluent2: paint window canvas via tree-anchored resource in template

When the theme is materialized as a deferred keyed resource (as ControlCatalog
declares it in App.xaml) the Window ControlTheme's Background styled setter
never resolves its DynamicResource, leaving the white transparency fallback
visible under semi-transparent page layers — dark mode composited to light
gray. Inner controls were unaffected because template-level DynamicResource
resolves through the visual tree.

Add a canvas border in the window template that paints
SolidBackgroundFillColorBaseBrush via the tree-anchored path; a user-set
Background still paints over it via the existing TemplateBinding border.
The styled-setter anomaly itself is an Avalonia core issue to investigate.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Avalonia.Themes.Fluent2/Controls/Window.xaml

@@ -15,6 +15,13 @@
       <ControlTemplate>
         <Panel>
           <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
+          <!-- Default canvas painted via a tree-anchored DynamicResource: styled-setter
+               DynamicResources on TopLevels have failed to resolve in hosted/reopened
+               window scenarios, leaving the white transparency fallback visible.
+               A user-set Background still paints over this via the border above it. -->
+          <Border Name="PART_Fluent2CanvasBackground"
+                  Background="{DynamicResource SolidBackgroundFillColorBaseBrush}"
+                  IsHitTestVisible="False" />
           <Border Background="{TemplateBinding Background}" IsHitTestVisible="False" />
           <Panel Background="Transparent" Margin="{TemplateBinding WindowDecorationMargin}" />
           <VisualLayerManager Name="PART_VisualLayerManager">