Use local deadline displays and live countdowns

[msaroufim]

Summary

Updates leaderboard deadline presentation so users see times in their browser timezone instead of a hard-coded UTC string.

Also updates homepage countdowns to refresh client-side once per second and switch to minutes/seconds when less than one hour remains, avoiding the unhelpful 0 days 0 hours remaining state near a deadline.

Details

• Adds toDateLocal for browser-local deadline formatting with a UTC fallback.
• Uses local deadline strings on leaderboard detail and editor headers.
• Makes getTimeLeft accept an explicit clock value and return minute/second precision under one hour.
• Adds a lightweight homepage interval to update countdown text and active/closed grouping without reloading.
• Narrows a homepage test link query now that the sponsor mail link is also present.

Validation

• npm test -- --run src/lib/date/utils.test.ts src/pages/home/Home.test.tsx
• npm run build