Add back some margins to the interface

There were a couple of cases where the lack of margins after the new layout changes just looked odd. Specifically, when the header is hidden (as in embedded mode), there would be no margin at the top of the window. Also the floating tile would run directly up against the sides of the window.

src/grid/GridLayout.module.css

@@ -40,7 +40,8 @@ limitations under the License.
   position: absolute;
   inline-size: 404px;
   block-size: 233px;
-  inset: 0;
+  inset-block: 0;
+  inset-inline: var(--cpd-space-3x);
 }
 
 .fixed > .slot[data-block-alignment="start"] {