[ui] Add a "Back to top" button that appears on scroll`

[AditthyaSS]

On long pages (models leaderboard, feed, model detail) there's no way to quickly return to the top without scrolling manually.

Expected behaviour:

Button appears in the bottom-right corner after scrolling down 400px
Smooth scrolls to the top on click
Fades in/out with a transition
Accessible: has aria-label="Back to top"
Implementation: Use useEffect with a scroll event listener — no library needed.

Files to create/touch:

New component:
BackToTop.tsx
layout.tsx
— add the component globally
Acceptance criteria:

Button hidden until user scrolls 400px
Smooth scroll to top on click
Fade in/out animation
Keyboard accessible and has aria-label
Works on mobile

[Muskankr]

Hi @AditthyaSS, I would like to work on this issue as NSOC'26 Contributor. Could you please assign this to me?