Add active states to top-level nav menu items

[1ockwood]

Changes

This is a fairly simple change that adds active states to the top-level nav menu items on large viewports. As a new user, the first time I clicked around, I expected to see that sort of feedback to give a sense of where I am in the app.

I kept it to the "desktop" version of the nav menu only, as the "mobile" menu is structured a bit differently (the top-level "Bookmarks" item being grouped at the same hierarchy as the other Bookmarks entries), and I don't necessarily feel it needs the same feedback in that context. Happy to experiment with that though if you feel otherwise.

In the same vein, I also tweaked the styles for hover states for the nav menu links.

Screenshots

Dark Mode

nav-menu-active-items-dark.mov

Light Mode

nav-menu-active-items-light.mov

[tkafka]

I love this! Any chance to merge?