Tweak control bar z-index to fix visual bug (#181)

Author: microbit-robert

src/components/control-bar/ControlBar.svelte

@@ -5,6 +5,6 @@
  -->
 
 <div
-  class="min-h-16 px-10 flex w-full items-center justify-between bg-secondary sticky top-0 z-4">
+  class="min-h-16 px-10 flex w-full items-center justify-between bg-secondary sticky top-0 z-5">
   <slot />
 </div>