Background: anchor logo to bottom-right, never overlap the footer

Two related issues with the background bitmap:

1. The footer (download progress bar) appeared on top of the image,
   cropping the bottom strip — including the ASTERION VR mark in the
   bottom-right corner of Background.png.

2. UniformToFill scales-and-crops to maintain aspect ratio, but with
   the previous setup the crop was centered, so resizing the window
   wider/narrower silently pushed the bottom-right logo off-screen.

Fix:
- The bitmap now lives in a Rectangle restricted to Grid.Row="1"
  (body only). When the footer appears at Row 2, it sits below the
  body without overlapping the image.
- Switch from <Image Stretch="UniformToFill"> to
  <Rectangle><Fill><ImageBrush AlignmentX="Right" AlignmentY="Bottom"
  Stretch="UniformToFill" /></Fill></Rectangle>. The crop now
  happens on the LEFT and TOP edges, keeping the logo's corner pinned
  to the bottom-right and visible regardless of window proportions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/PSLauncher.App/Views/MainWindow.xaml

@@ -166,15 +166,20 @@
             <RowDefinition Height="Auto" />
         </Grid.RowDefinitions>
 
-        <!-- Background image (sous tout le reste, ne capte pas la souris).
+        <!-- Background image : limité au body uniquement (Grid.Row="1") pour ne pas
-             Le fond noir du Grid garantit que les zones transparentes de l'image
+             se faire chevaucher par le footer quand un download tourne. ImageBrush
-             (et l'image dans son ensemble à faible opacity) reposent sur du noir
+             avec AlignmentX=Right + AlignmentY=Bottom : si le ratio fenêtre force
-             pur, sans héritage du brush système. -->
+             un crop, on rogne en haut/à gauche — le logo ASTERION VR en bas-droite
-        <Image Grid.Row="0" Grid.RowSpan="3"
+             reste TOUJOURS visible. -->
-               Source="pack://application:,,,/Resources/Background.png"
+        <Rectangle Grid.Row="1" IsHitTestVisible="False">
+            <Rectangle.Fill>
+                <ImageBrush ImageSource="pack://application:,,,/Resources/Background.png"
                             Stretch="UniformToFill"
-               Opacity="0.25"
+                            AlignmentX="Right"
-               IsHitTestVisible="False" />
+                            AlignmentY="Bottom"
+                            Opacity="0.25" />
+            </Rectangle.Fill>
+        </Rectangle>
 
         <!-- Top bar : 3 colonnes (brand / license center / chrome) -->
         <Border Grid.Row="0" Background="{StaticResource Brush.Bg.Sidebar}"