fix(socials): live preview of the bar appearance + the crash on dragging a slider

Two bugs from the creator's first look at the new controls.

1. CRASH — '{DependencyProperty.UnsetValue}' is not a valid value for property 'Fill'.
   The token conversion deleted `YtSliderKnobHoverBrush` while two slider triggers still
   referenced it. WPF does not fail that at build time: it substitutes UnsetValue and throws at
   the next layout, inside `Ellipse.OnRender` — so the first slider touched (the new opacity one)
   killed the app. The brush is back, as a glossier version of the rest state.
   Guard: `ThemeResourceTests` scans every app XAML and proves each {StaticResource} key is
   defined (theme/App resources or the same file). Proven by removing the key again — the test
   names it.

2. PREVIEW DID NOTHING — the preview bar is its own XAML rendering (not the rasterized strip the
   compositor overlays), and it was hard-wired white-on-transparent, so the drawer's controls had
   nothing to move. Now:
   - `SocialBarTextBrush` / `SocialBarBackgroundBrush` come from the same `SocialBarStyle` the
     renderer bakes, so preview and output cannot disagree.
   - the preview draws the plate (same colour + opacity) and the drop shadow.
   - the drawer applies changes to the live config as you edit (`ApplyAppearancePreview`), and
     Cancel restores the pre-edit snapshot — so the preview tracks the controls without
     persisting anything until Save.

46 tests green (theme, socials, design tokens, layout store, real-MainWindow meter test).

ViewModels/MainViewModel.Socials.cs

@@ -31,6 +31,13 @@ public partial class MainViewModel
     /// <summary>Green glow on the bar while it's on the stream.</summary>
     public SolidColorBrush SocialBarGlowBrush => BarOnBrush;
 
+    /// <summary>Glyph colour of the bar — the preview draws the same value the renderer paints,
+    /// 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 global socials config (entries + bar settings), or null.</summary>
     public SocialsConfig? Socials => _socials;
 
@@ -137,13 +144,28 @@ public partial class MainViewModel
             SignOutYouTubeAsync,
             _youtubeAuth.CurrentChannel,
             _socials);
+        // Live preview: the drawer edits the REAL config, so colour/opacity/shadow land on the
+        // preview immediately instead of waiting for Save. Cancel restores the snapshot below.
+        _socialsBeforeEdit = _socials;
+        dialog.PropertyChanged += (_, e) =>
+        {
+            if (e.PropertyName is null) return;
+            if (!IsAppearanceProperty(e.PropertyName)) return;
+            ApplyAppearancePreview(dialog);
+        };
         dialog.SaveRequested += () =>
         {
             _socials = dialog.BuildSocialsConfig();
             NotifySocialsChanged();
             CloseSocialsDrawer();
         };
-        dialog.CancelRequested += CloseSocialsDrawer;
+        dialog.CancelRequested += () =>
+        {
+            _socials = _socialsBeforeEdit;
+            NotifySocialsChanged();
+            OnPropertyChanged(nameof(SocialBarVisible));
+            CloseSocialsDrawer();
+        };
         SocialsDrawer = dialog;
         BroadcastForm.IsDrawerOpen = false;
         Ypp.IsDrawerOpen = false;
@@ -153,6 +175,28 @@ public partial class MainViewModel
         OnPropertyChanged(nameof(SocialsDrawerOpen));
     }
 
+    private SocialsConfig? _socialsBeforeEdit;
+
+    private static bool IsAppearanceProperty(string name) => name is
+        nameof(SocialsDialogViewModel.BarEnabled) or nameof(SocialsDialogViewModel.BarTextColor)
+        or nameof(SocialsDialogViewModel.BarBackgroundEnabled) or nameof(SocialsDialogViewModel.BarBackgroundColor)
+        or nameof(SocialsDialogViewModel.BarBackgroundOpacity) or nameof(SocialsDialogViewModel.BarTextShadow);
+
+    /// <summary>Paints the drawer's current appearance onto the live config and re-renders the
+    /// strip. Nothing is persisted here — Save writes, Cancel restores the snapshot.</summary>
+    private void ApplyAppearancePreview(SocialsDialogViewModel dialog)
+    {
+        if (_socials == null) return;
+        _socials.BarEnabled = dialog.BarEnabled;
+        _socials.BarTextColor = dialog.BarTextColor;
+        _socials.BarBackgroundEnabled = dialog.BarBackgroundEnabled;
+        _socials.BarBackgroundColor = dialog.BarBackgroundColor;
+        _socials.BarBackgroundOpacity = dialog.BarBackgroundOpacity;
+        _socials.BarTextShadow = dialog.BarTextShadow;
+        NotifySocialsChanged();
+        OnPropertyChanged(nameof(SocialBarVisible));
+    }
+
     internal void CloseSocialsDrawer()
     {
         if (!SocialsDrawerOpen) return;
@@ -164,6 +208,8 @@ public partial class MainViewModel
     private void NotifySocialsChanged()
     {
         RenderSocialBarFrame();
+        OnPropertyChanged(nameof(SocialBarTextBrush));
+        OnPropertyChanged(nameof(SocialBarBackgroundBrush));
         OnPropertyChanged(nameof(HasSocials));
         OnPropertyChanged(nameof(SocialBarVisible));
         OnPropertyChanged(nameof(SocialBarDotBrush));