- Add explicit close (×) button inside mobile nav overlay
- Was unreachable because header's hamburger was behind overlay z-index
- New button lives inside the overlay at full z-index visibility
- Fix check-forms.ts: authenticate via Gatekeeper login form
- Old approach: inject raw password as session cookie (didn't work)
- New approach: navigate to protected page, detect Gatekeeper gate,
fill password form and submit to get a real server-signed session cookie
- Fixes E2E form tests that failed because pages returned Gatekeeper HTML
