Add scroll-aware back-to-top button

components/back-to-top.tsx

@@ -0,0 +1,37 @@
+'use client';
+
+import { useEffect, useState } from 'react';
+
+export function BackToTop() {
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    const onScroll = () => {
+      if (window.scrollY > 400) {
+        setVisible(true);
+      } else {
+        setVisible(false);
+      }
+    };
+
+    onScroll();
+    window.addEventListener('scroll', onScroll, { passive: true });
+    return () => window.removeEventListener('scroll', onScroll);
+  }, []);
+
+  if (!visible) return null;
+
+  return (
+    <button
+      type="button"
+      onClick={() => {
+        window.scrollTo({ top: 0, behavior: 'smooth' });
+      }}
+      aria-label="回到頁面頂部"
+      className="fixed bottom-6 right-4 z-40 inline-flex h-9 w-9 items-center justify-center rounded-full bg-slate-900 text-slate-50 shadow-md ring-1 ring-slate-800/70 transition hover:bg-slate-700 dark:bg-slate-100 dark:text-slate-900 dark:ring-slate-300/70 dark:hover:bg-slate-300"
+    >
+      <span className="text-lg leading-none">↑</span>
+    </button>
+  );
+}
+