fix(socials): the bar disappeared from the preview when the plate was off

My own regression from the previous commit: the plate Border WRAPPED the bar's content and its
Visibility was bound to BarBackgroundEnabled — so with the plate off (the default) the preview
collapsed the entire social bar. Nothing was wrong with the data; the element was there and
invisible.

Fix: the wrapper is always laid out. "Plate off" now means a TRANSPARENT background
(`SocialBarBackgroundBrush` returns Transparent when disabled) and zero padding, "on" means the
colour/opacity plate with 18,6 padding — which also matches what the renderer bakes for the
output, so preview and broadcast agree.

Guard (new fact in SocialBarAppearanceTests): the preview must never visibility-gate the bar on
BarBackgroundEnabled, the wrapper must bind SocialBarBackgroundBrush, and the plate's switching
must be Padding.

Controls/PreviewPane.xaml

@@ -351,10 +351,18 @@
                                             <DropShadowEffect Color="{Binding SocialBarGlowBrush.Color}"
                                                               BlurRadius="18" ShadowDepth="0" Opacity="0.9"/>
                                         </Grid.Effect>
-                                        <Border HorizontalAlignment="Center" Margin="40,8"
-                                                CornerRadius="20" Padding="18,6"
-                                                Background="{Binding SocialBarBackgroundBrush}"
-                                                Visibility="{Binding Socials.BarBackgroundEnabled, Converter={StaticResource BoolToVis}}">
+                                        <Border HorizontalAlignment="Center" Margin="40,8" CornerRadius="20"
+                                                Background="{Binding SocialBarBackgroundBrush}">
+                                            <Border.Style>
+                                                <Style TargetType="Border">
+                                                    <Setter Property="Padding" Value="0"/>
+                                                    <Style.Triggers>
+                                                        <DataTrigger Binding="{Binding Socials.BarBackgroundEnabled}" Value="True">
+                                                            <Setter Property="Padding" Value="18,6"/>
+                                                        </DataTrigger>
+                                                    </Style.Triggers>
+                                                </Style>
+                                            </Border.Style>
                                         <ItemsControl ItemsSource="{Binding Socials.Entries}"
                                                       HorizontalAlignment="Center"
                                                       IsHitTestVisible="False">