fix: rework mobile nav as fixed overlay, add 3-line burger

- Nav menu is now a fixed fullscreen overlay (opacity fade)
- Header stays at 3.5rem height, no height animation
- Burger has 3 lines, middle fades out on open
- Body scroll locked with overflow: hidden when menu open
- Various mobile layout fixes (handles, system, footer padding)
- Header full-width with no border-radius on mobile

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

index.html

@@ -54,6 +54,7 @@
         <button class="burger" aria-label="Menu">
           <span></span>
           <span></span>
+          <span></span>
         </button>
         <ul>
           <li>
@@ -409,7 +410,7 @@
 
       <section id="author">
         <img
-          src="/assets/images/frederic-denjoy.jpg"
+          src="/assets/images/frederic-denjoy.jpeg"
           alt="Portrait de l'auteur Frédéric Denjoy"
         />
         <div class="text">