feature: Add custom branded not-found.tsx page for 404 routes #271

When a visitor navigates to an invalid route (e.g. /unknown-path or a broken link), Next.js displays the default unstyled 404 page. DevImpact has UserNotFoundCard for missing developer handles, but lacks a root src/app/not-found.tsx.

Motivation

A branded 404 page keeps visitors on the site, preserves header/footer navigation, and provides clear routes back to the comparison dashboard and leaderboards.

Tasks

• Create src/app/not-found.tsx using AppHeader and AppFooter.
• Display BrandLogo and a clean 404 status card.
• Provide 'Back to Home' and 'Browse Leaderboards' action buttons.
• Add translation keys notfound.title and notfound.description to en.json and ar.json.

Files to Check

• src/app/not-found.tsx
• src/locales/en.json & src/locales/ar.json