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/PhotoSwipe.astro

@@ -10,6 +10,8 @@
       lightbox.destroy()
       lightbox = null
     }
+    document.removeEventListener('astro:page-load', createPhotoSwipe)
+    document.removeEventListener('astro:before-swap', cleanup)
   }
 
   function createPhotoSwipe() {