Footer : drop the leftover Margin=200 hack

The 200px left margin was a workaround when the footer was at outer
Row=2 with no column constraint and had to dodge the sidebar visually.
Now that Row=2 + Col=1 cleanly cantons the footer in the content
column (commit ab40b62), the margin pushes everything 200px further
right and creates a black gap at the bottom-left of the content area.

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

src/PSLauncher.App/Views/MainWindow.xaml

@@ -647,7 +647,6 @@
                 Background="{StaticResource Brush.Bg.Footer}"
                 BorderBrush="{StaticResource Brush.Border}" BorderThickness="0,1,0,0"
                 Padding="20,10"
-                Margin="200,0,0,0"
                 Visibility="{Binding FooterVisibility}">
             <Grid>
                 <Grid.ColumnDefinitions>