feat: add mobile burger menu, SEO meta tags, and various fixes

- Add burger menu with open/close animation and scroll lock
- Add nav-logo element for mobile header
- Hide nav links on mobile, reveal in fullscreen overlay
- Add title, meta description, Open Graph and Twitter Card tags
- Add canonical URL
- Various SCSS adjustments (hero line-height, excerpt font-weight)
- Fix asset paths

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

assets/js/script.js

@@ -1,4 +1,23 @@
-document.addEventListener('DOMContentLoaded', function() {
+document.addEventListener('DOMContentLoaded', function () {
+  // ---- MOBILE NAV ----
+  const header = document.querySelector('body > header');
+  const burger = header.querySelector('.burger');
+  const navLinks = header.querySelectorAll('nav ul a');
+
+  burger.addEventListener('click', function () {
+    header.classList.toggle('nav-open');
+    document.body.classList.toggle('no-scroll');
+  });
+
+  navLinks.forEach(function (link) {
+    link.addEventListener('click', function () {
+      if (!header.classList.contains('nav-open')) return;
+      header.classList.remove('nav-open');
+      document.body.classList.remove('no-scroll');
+    });
+  });
+
+
   const excerptSwiper = new Swiper('.excerpts-swiper', {
     // Slides visibles et groupement
     slidesPerView: 3,