fix: position playground panel relative to header

Author: DylanPiercey

src/routes/playground/tags/split-panes.marko

@@ -41,7 +41,7 @@ style --
   .panes {
     display: flex;
     position: fixed;
-    inset: 8rem 0 0 0;
+    inset: var(--stick-size) 0 0 0;
 
     > * {
       flex-basis: 0;