Bring back silky-smooth nav animations

[nedtwigg]

As of this commit

• f653996
• https://7b215f1c.gitfromscratch.pages.dev/branches/sticky-notes-and-paintbrushes

The nav did a cool animation to subitems on a page.

It has problems because the items can now be multi-line, the arithmetic for where to move the background highlight is tricky.

So we removed it in

• Less ideal but more stable active nav link highlight #17
• https://e5d5819c.gitfromscratch.pages.dev/

It would be cool to add it back someday, but it's not high-priority.