Revert "Migrate to HeroUI v3 and Tailwind CSS v4"

This reverts commit 6a9296f33d.

components/back-to-top.tsx

@@ -1,7 +1,6 @@
 'use client';
 
 import { useEffect, useState } from 'react';
-import { Button } from '@heroui/react';
 
 export function BackToTop() {
   const [visible, setVisible] = useState(false);
@@ -23,16 +22,16 @@ export function BackToTop() {
   if (!visible) return null;
 
   return (
-    <Button
-      isIconOnly
-      variant="secondary"
-      className="fixed bottom-6 right-4 z-40 h-9 w-9 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"
-      onPress={() => {
+    <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>
+    </button>
   );
 }
+