[FEATURE] Add scroll-to-top button

[ascender1729]

Description

Add a floating button that appears when scrolled down to quickly return to the top of the page.

Why Needed

• Long result lists require scrolling
• Quick navigation back to search
• Common UX pattern

Implementation Details

• Show button after scrolling 300-500px
• Fixed position bottom-right
• Smooth scroll animation to top
• Fade in/out transition

Files to Create/Modify

• src/components/shared/ScrollToTop.svelte (new)
• src/layouts/Layout.astro or main component

Code Example

<script>
  let showButton = false;
  
  function handleScroll() {
    showButton = window.scrollY > 400;
  }
  
  function scrollToTop() {
    window.scrollTo({ top: 0, behavior: 'smooth' });
  }
</script>

<svelte:window on:scroll={handleScroll} />

{#if showButton}
  <button onclick={scrollToTop} class="fixed bottom-4 right-4">
    <ArrowUpIcon />
  </button>
{/if}

Acceptance Criteria

• Button appears when scrolled down
• Smooth scroll animation to top
• Fade in/out transition
• Mobile-friendly size and position

Time Estimates

• With AI: 135 minutes
• Without AI: 900 minutes