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/components/Scrollbar.astro

@@ -41,7 +41,6 @@ document.addEventListener('astro:before-swap', () => {
   scrollbarsInstance = null
 })
 
-document.addEventListener('DOMContentLoaded', initScrollbar)
 document.addEventListener('astro:page-load', initScrollbar)
 </script>