Extract navbar and footer into shared header.html and footer.html partials

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

src/main.js

@@ -1 +1,21 @@
+function loadHTML(file, elementId) {
+  fetch(file)
+    .then(r => r.text())
+    .then(html => {
+      document.getElementById(elementId).innerHTML = html;
+      if (elementId === 'header') highlightActiveNav();
+    })
+    .catch(err => console.error('Error loading:', file, err));
+}
 
+function highlightActiveNav() {
+  const current = location.pathname.split('/').pop() || 'index.html';
+  document.querySelectorAll('#header a[href]').forEach(link => {
+    if (link.getAttribute('href') === `/${current}`) {
+      link.classList.add('active');
+    }
+  });
+}
+
+loadHTML('/header.html', 'header');
+loadHTML('/footer.html', 'footer');