Add contributor leaderboard component to dashboard

[AbdulSnk]

Overview

The maintainer dashboard needs a dedicated contributor leaderboard component that visually ranks contributors by their StreamForge score in real time.

What needs to be done

• Create a reusable ContributorLeaderboard component under src/components/
• Display the following for each contributor:
  • Rank number
  • Contributor GitHub username and avatar
  • Total score
  • Number of contributions (PRs, commits, reviews)
  • Total rewards earned in USDC
• Fetch data from the StreamForge API scoring endpoint
• Support pagination or a "load more" button if contributors exceed 10
• Highlight the top 3 contributors visually (e.g. gold, silver, bronze)

Acceptance Criteria

• ContributorLeaderboard component exists under src/components/
• Displays all fields listed above per contributor
• Top 3 contributors are visually distinguished
• Pagination or "load more" is implemented
• Component handles loading, empty, and error states
• Component is integrated into the /dashboard route

Notes

Keep the component fully reusable — it should work as a standalone component that can be dropped into any page. GitHub avatars can be fetched using https://github.com/{username}.png.