fix(website): custom 404 page returns HTTP 200 status code

## Problem
Navigating to a non-existent URL (e.g. /this-does-not-exist) displays the 404 error page but returns HTTP 200 status. This is a 'soft 404' which confuses search engines and breaks HTTP semantics.

## Fix
Ensure `not-found.tsx` in Next.js App Router uses `notFound()` from `next/navigation` which correctly sets the 404 status code. Check `website/src/app/not-found.tsx`.