feat: accessibility for site-menu

- #menu-toggle: aria-expanded, aria-controls, aria-label (updated on toggle)
- #site-menu: inert when closed (blocks focus + screen readers), removed when open

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

site/snippets/header.php

@@ -80,7 +80,7 @@
         <?php endforeach ?>
     </div>
 
-    <button id="menu-toggle">
+    <button id="menu-toggle" aria-expanded="false" aria-controls="site-menu" aria-label="Ouvrir le menu">
       <span class="open"><?= svg('assets/icons/menu.svg') ?></span>
       <span class="close"><?= svg('assets/icons/close.svg') ?></span>
     </button>

site/snippets/nav.php

@@ -1,4 +1,4 @@
-<aside id="site-menu" aria-label="Menu principal">
+<aside id="site-menu" aria-label="Menu principal" inert>
     <div class="site-menu__inner">
 <p class="baseline">Index est une ONG d’investigation numérique au service du public,
 de la vérité et de la justice.</p>