refactor: optimize website performance and layout

- Replace ScrollButton with BackToTop component
- Improve theme toggle and scrollbar event handling
- Update layout and index page styling
- Optimize client:load and client:idle directives
- Remove deprecated Heti addon plugin

src/pages/[lang]/index.astro

@@ -47,7 +47,7 @@ const postsByYear = await getPostsByYear(lang)
               {/* Post Title */}
               <a href={`/${lang}/posts/${post.data.slug || post.slug}`}>{post.data.title}</a>
                 {/* Post Date */}
-                <time class="block text-5.6 leading-7 font-navbar opacity-30 lg:inline lg:before:content-['_']" aria-hidden="true">
+                <time class="block text-5.6 leading-7 font-navbar opacity-30 lg:inline lg:before:content-['_']">
                   {post.data.published.toLocaleDateString('en-US', { month: '2-digit', day: '2-digit' }).replace('/', '-')}
                   {post.remarkPluginFrontmatter?.minutes && <span class="ml-2">  {post.remarkPluginFrontmatter.minutes} min</span>}
                 </time>