fix: Mispositioned TOC control on mobile due to merge conflict

fix: Show message in mobile TOC when no headings in document
fix: MenuItem with level should still have background edge-to-edge
fix: Show developer warning when creating incorrect menu item type

app/components/Header.js

@@ -72,8 +72,6 @@ const Actions = styled(Flex)`
   flex-basis: 0;
   min-width: auto;
   padding-left: 8px;
-  position: fixed;
-  right: 12px;
 
   ${breakpoint("tablet")`
     position: unset;