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/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