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/css/src/_header.scss

@@ -8,6 +8,11 @@ body {
     padding: 1.5rem var(--padding-body);
     box-sizing: border-box;
 
+    .nav-logo,
+    .burger {
+      display: none;
+    }
+
     ul {
       display: flex;
       justify-content: space-between;