fix: close mobile nav when tapping outside the menu

[braboj]

Summary

• Add document-level click listener that closes the mobile navigation when tapping outside the hamburger button and nav panel
• Fixes When I open the navigation and tip outside it, the navigation is not closed on mobile #171

Test plan

• Open site on mobile (or responsive mode at ≤768px)
• Tap hamburger to open nav
• Tap outside the nav panel — nav should close
• Tap hamburger to open, then tap a nav link — navigation should work normally
• On desktop (>768px) verify no change in behavior