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>

assets/css/base/_body.scss

@@ -139,10 +139,6 @@ body, #site-header, #site-footer{
 body.menu-open,
 body.is-hidden{
     overflow-y: hidden;
-    main, footer, #nav-highlight, .btn--back-to-top, .bottom-bar__inner{
-        transition: opacity .3s ease-in;
-        opacity: 0.1;
-    }
 }