[6.x] Fix mobile nav button failing

[jaygeorge]

Description of the Problem

If you pull the window in so the nav switches to a burger state, it sometimes fails to function on page load.

The nav talked to the sidebar through Statamic.$events, which was flagged as flaky (click/timing races, especially on mobile), and the drawer could flash on screen before JS added the right classes.

Nav.Failing.mp4

What this PR Does

• Uses a small direct “call this function” hook between Logo and Nav instead of events
• Ignore outside-clicks on the burger
• CSS changes to keep the mobile drawer off-screen before JS runs.
• Also fixes the nav overflow height, taking the dynamic address bar UI into consideration

To be honest, I think this nav should be a popover, which would be less fragile and drastically simplify things – but that's probably an even bigger change, and something to consider in future if we start changing the component.

You can see the behaviour is fixed now:

Nav.Working.mp4

How to Reproduce

1. Make the viewport narrow so you can see the nav burger
2. Make sure DevTools are closed so caching is not force-cleared on load
3. Toggle the nav in
4. Reload the page
5. Try toggling the nav again. It will not work until you load the page a third time.