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).

Themes/Controls.xaml

@@ -359,6 +359,16 @@
         <GradientStop Color="{StaticResource Color.Tertiary}" Offset="1"/>
     </RadialGradientBrush>
 
+    <!-- Hover/drag knob: the same gloss with the highlight spread wider. The conversion to
+         tokens deleted this key while two triggers still referenced it, and WPF turns a missing
+         StaticResource into DependencyProperty.UnsetValue — which crashes Ellipse.OnRender at
+         the next layout. ThemeResourceTests now catches that class of defect. -->
+    <RadialGradientBrush x:Key="YtSliderKnobHoverBrush" Center="0.35,0.3" GradientOrigin="0.35,0.3">
+        <GradientStop Color="#ffffff" Offset="0"/>
+        <GradientStop Color="#ffffff" Offset="0.3"/>
+        <GradientStop Color="{StaticResource Color.Tertiary}" Offset="1"/>
+    </RadialGradientBrush>
+
     <Style x:Key="YtSliderTrackButton" TargetType="RepeatButton">
         <Setter Property="Focusable" Value="False"/>
         <Setter Property="Template">