fix(dock): spans the preview column and loses 20px

- The dock now sits in the preview column (Grid.Column=1), so its left edge is the layers panel's
  right edge instead of starting under the brand column. It is a preview-column control, not a
  full-width footer.
- Expanded height 168 → 148 (112 bays + 36 master strip): the mockup's number read as wasted space
  beneath the 154px scene strip.

Guard: AudioDockLayoutTests pins 148 as the design number, so the constant cannot drift silently.

ytLive.Tests/AudioDockLayoutTests.cs

@@ -26,7 +26,7 @@ public class AudioDockLayoutTests
         Assert.True(AudioDockLayout.IsCollapsed(AudioDockLayout.CollapseBelowHeight - 1));
 
         // The two heights are the design's numbers, not whatever looked fine.
-        Assert.Equal(168, AudioDockLayout.ExpandedHeight);
+        Assert.Equal(148, AudioDockLayout.ExpandedHeight);   // 112 bays + 36 strip
         Assert.Equal(36, AudioDockLayout.CollapsedHeight);
     }
 }