Fix deterministic leaderboard sorting and tied ranks

[nikkkkj]

Fixes #8.

Summary

• Extract leaderboard sorting/ranking into a reusable helper
• Sort by earned amount descending, then name/id for deterministic ties
• Assign equal ranks to users with the same earnings
• Add node:test coverage for tie ordering and rank assignment
• Fix an existing form validation TypeScript error that blocked production builds

Verification

• npm test
• npm run build

Note: npm run build still reports the existing Next.js <img> lint warning in src/components/leaderboard.tsx, but the build completes successfully.