make escape close navigation on narrow screen

[Alex-Jordan]

On a narrow screen, when the sidebar navigation is open, this change makes it so that the escape key closes the navigation.

From Claude:

This also replaced the ad hoc backdrop
creation/removal scattered across the click handler, resize handler, and
inline toggle logic with a single openMobileDrawer/closeMobileDrawer pair,
which incidentally fixes a latent bug where clicking the toggle button while
the drawer was already open left a stale extra backdrop in the DOM.

This was all a "medium" accessibility issue that Claude found.

[drgrice1]

I just noticed something else that will probably need to be made inert while the sidebar is open on a narrow screen. That is the "Skip to main content" link. I noticed that if the sidebar is open, that link is still in the tab order, and if you focus it with the keyboard and hit enter it still attempts to focus the page content, and then things get weird with the tab order.

[Alex-Jordan]

OK, I made some changes here that I think address your comments.