feat: Stable Ranking and Deterministic Sorting for Leaderboard (#2, #8)

[TodouWisiper]

🏆 Stable Ranking & Deterministic Sorting Implementation

This PR addresses issue #2 and #8 by fixing the unstable sorting logic and implementing proper competition ranking.

✨ Key Improvements:

1. Deterministic Sorting: Added secondary and tertiary sort keys.
   • 1st: Total Earned (Descending)
   • 2nd: Bounties Completed (Descending)
   • 3rd: Name (Alphabetical)
     This ensures users don't jump around on every refresh when earnings are tied.
2. Standard Competition Ranking (1-2-2-4): Users with the same earnings now correctly receive the same rank, following the standard competition model.
3. Enhanced UI:
   • Added Medal Emojis (🥇, 🥈, 🥉) for the top 3 earners.
   • Improved card styling with better hover states, borders, and rounded corners.
   • Added a clear "Total Earned" label for better data visualization.
4. Performance: Wrapped logic in useMemo to prevent unnecessary recalculations on re-renders.

🧪 Verification:

• Tied users Alice and Bob (both $35.00) now correctly share Rank 2.
• Tied users Diana, Eve, and Frank (all $20.00) now correctly share Rank 4.
• Sorting remains consistent across all renders.

/claim #2
/claim #8