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.

ViewModels/MainViewModel.Socials.cs

@@ -35,8 +35,11 @@ public partial class MainViewModel
     /// so the two cannot drift apart (creator request 2026-10-30).</summary>
     public SolidColorBrush SocialBarTextBrush => SocialBarStyle.From(_socials).TextBrush;
 
-    /// <summary>The preview's plate: same colour AND same opacity the renderer bakes in.</summary>
-    public SolidColorBrush SocialBarBackgroundBrush => SocialBarStyle.From(_socials).BackgroundBrush;
+    /// <summary>The preview's plate: same colour AND same opacity the renderer bakes in.
+    /// Transparent when the plate is off — "off" means no plate, never no bar.</summary>
+    public SolidColorBrush SocialBarBackgroundBrush => _socials?.BarBackgroundEnabled == true
+        ? SocialBarStyle.From(_socials).BackgroundBrush
+        : Brushes.Transparent;
 
     /// <summary>The global socials config (entries + bar settings), or null.</summary>
     public SocialsConfig? Socials => _socials;