feat(website): add back-to-top button

Adds a fixed-position button that fades in after scrolling 600px and
smoothly scrolls back to the top on click. Hidden by default via the
HTML hidden attribute so noscript users never see it.

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

website/static/main.js

@@ -279,6 +279,25 @@ if (searchInput) {
   });
 }
 
+// Back to top
+var backToTop = document.querySelector('.back-to-top');
+if (backToTop) {
+  backToTop.hidden = false;
+  var scrollTicking = false;
+  window.addEventListener('scroll', function () {
+    if (!scrollTicking) {
+      requestAnimationFrame(function () {
+        backToTop.classList.toggle('visible', window.scrollY > 600);
+        scrollTicking = false;
+      });
+      scrollTicking = true;
+    }
+  });
+  backToTop.addEventListener('click', function () {
+    window.scrollTo({ top: 0, behavior: 'smooth' });
+  });
+}
+
 // Restore state from URL
 (function () {
   var params = new URLSearchParams(location.search);

website/static/style.css

@@ -527,6 +527,51 @@ th[data-sort].sort-asc::after {
   font-weight: 600;
 }
 
+/* === Back to Top === */
+.back-to-top {
+  position: fixed;
+  bottom: 2rem;
+  right: max(1.5rem, calc(50vw - 700px + 0.5rem));
+  background: var(--bg);
+  border: 1px solid var(--border-strong);
+  border-radius: 4px;
+  padding: 0.4rem 0.75rem;
+  font-family: var(--font-body);
+  font-size: var(--text-xs);
+  font-weight: 600;
+  color: var(--text-muted);
+  cursor: pointer;
+  opacity: 0;
+  transform: translateY(0.5rem);
+  transition: opacity 0.25s ease, transform 0.25s ease, border-color 0.15s, color 0.15s;
+  z-index: 50;
+}
+
+.back-to-top[hidden] { display: none; }
+
+.back-to-top.visible {
+  opacity: 1;
+  transform: translateY(0);
+}
+
+.back-to-top:hover {
+  border-color: var(--accent);
+  color: var(--accent);
+}
+
+.back-to-top:active {
+  transform: scale(0.95);
+}
+
+.back-to-top:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
+}
+
+@media (max-width: 640px) {
+  .back-to-top { right: 1.25rem; bottom: 1.25rem; }
+}
+
 /* === Noscript === */
 .noscript-msg {
   text-align: center;

website/templates/base.html

@@ -37,6 +37,8 @@
 
     <main id="content">{% block content %}{% endblock %}</main>
 
+    <button class="back-to-top" aria-label="Back to top" hidden>&uarr; Top</button>
+
     <footer class="footer">
       <span
         >Made by