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.

Models/AudioDockLayout.cs

@@ -2,7 +2,7 @@ namespace ytLive.Models;
 
 /// <summary>How tall the audio dock should be for a given window height.
 ///
-/// <para>Creator ruling (2026-10-30): the dock is 168px on a normal window and collapses to a 36px
+/// <para>Creator ruling (2026-10-30): the dock is 148px on a normal window and collapses to a 36px
 /// telemetry strip when the window is short, because at 1366×768 a full dock leaves ~398px for the
 /// preview and the composition starts letterboxing into a postage stamp.</para>
 ///
@@ -13,7 +13,9 @@ public static class AudioDockLayout
     /// default window is 1040, so a normal launch shows the full dock.</summary>
     public const double CollapseBelowHeight = 900;
 
-    public const double ExpandedHeight = 168;
+    /// <summary>148 = 112 of bays + the 36px master strip. The mockup wanted 168; the creator
+    /// read that as wasted space under a 154px scene strip, so the bays lost 20px of air.</summary>
+    public const double ExpandedHeight = 148;
     public const double CollapsedHeight = 36;
 
     public static bool IsCollapsed(double windowHeight) => windowHeight < CollapseBelowHeight;