feat: replace menu opacity hack with proper overlay

- Add #menu-overlay div (fixed, full screen, z-index below menu)
- Overlay fades in/out via opacity+visibility transition (cursor: pointer)
- Click on overlay closes the menu
- Remove opacity: 0.1 hack on main/footer/etc. when menu-open

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

site/snippets/header.php

@@ -89,3 +89,4 @@
   </div>
 </header>
 <?php snippet('nav') ?>
+<div id="menu-overlay"></div>