Force black background under bg image, Launcher subtitle bold

The implicit Window style in Theme.xaml WAS being applied, but defensive
coding wins: when MainWindow.Background was unset, WPF's intermediate
composition surface for elements with Opacity<1 (the Image) blended
against whatever the parent presented. In some Windows render paths
that's effectively the system white. Setting Window.Background="Black"
AND Grid.Background="Black" pins the surface under the image to pure
black explicitly, no system theme involvement.

Also drop the previous "darken overlay" Rectangle — once the surface
under the image is genuinely black, the image at 0.25 opacity is
already where we want it, no second dimming layer needed.

Launcher subtitle: FontWeight Light → Bold so it competes equally with
PROSERVE in the Pirulen wordmark.

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

src/PSLauncher.App/Views/MainWindow.xaml

@@ -3,6 +3,7 @@
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:vm="clr-namespace:PSLauncher.App.ViewModels"
         Title="PROSERVE Launcher"
+        Background="Black"
         Width="980" Height="780"
         MinWidth="780" MinHeight="500"
         WindowStartupLocation="CenterScreen"
@@ -145,22 +146,21 @@
         </DataTemplate>
     </Window.Resources>
 
-    <Grid>
+    <Grid Background="Black">
         <Grid.RowDefinitions>
             <RowDefinition Height="Auto" />
             <RowDefinition Height="*" />
             <RowDefinition Height="Auto" />
         </Grid.RowDefinitions>
 
-        <!-- Background image (sous tout le reste, ne capte pas la souris) -->
+        <!-- Background image (sous tout le reste, ne capte pas la souris).
+             Le fond noir du Grid garantit que les zones transparentes de l'image
+             (et l'image dans son ensemble à faible opacity) reposent sur du noir
+             pur, sans héritage du brush système. -->
         <Image Grid.Row="0" Grid.RowSpan="3"
                Source="pack://application:,,,/Resources/Background.png"
                Stretch="UniformToFill"
-               Opacity="0.18"
-               IsHitTestVisible="False" />
-        <!-- Voile noir semi-opaque par-dessus pour assombrir encore et garantir la lisibilité -->
-        <Rectangle Grid.Row="0" Grid.RowSpan="3"
-                   Fill="Black" Opacity="0.35"
+               Opacity="0.25"
                IsHitTestVisible="False" />
 
         <!-- Top bar -->
@@ -176,7 +176,7 @@
                                VerticalAlignment="Center" />
                     <TextBlock Text="Launcher"
                                FontSize="20"
-                               FontWeight="Light"
+                               FontWeight="Bold"
                                Foreground="{StaticResource Brush.Text.Primary}"
                                VerticalAlignment="Center"
                                Margin="14,0,0,0" />