✨feature: Add responsive styles

app.js

@@ -106,3 +106,19 @@ async function resetGame() {
 }
 
 window.addEventListener('DOMContentLoaded', onLoad);
+
+// Back to Top: show after scrolling, smooth-scroll on click
+function initBackToTop() {
+  const btn = document.getElementById('backToTop');
+  if (!btn) return;
+  const onScroll = () => {
+    if (window.scrollY > 200) btn.classList.add('show'); else btn.classList.remove('show');
+  };
+  window.addEventListener('scroll', onScroll, { passive: true });
+  onScroll();
+  btn.addEventListener('click', () => {
+    window.scrollTo({ top: 0, behavior: 'smooth' });
+  });
+}
+
+window.addEventListener('DOMContentLoaded', initBackToTop);