first commit

public/js/scroll-to-top.js

@@ -0,0 +1,14 @@
+export function bindScrollToTop(button) {
+  if (!button) return;
+
+  function updateVisibility() {
+    button.hidden = window.scrollY <= 480;
+  }
+
+  window.addEventListener('scroll', updateVisibility, { passive: true });
+  button.addEventListener('click', () => {
+    const reduceMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
+    window.scrollTo({ top: 0, behavior: reduceMotion ? 'auto' : 'smooth' });
+  });
+  updateVisibility();
+}