[Airdrop P3] Leaderboard component

[realproject7]

Parent: #877
Depends on: #885

Scope

Leaderboard showing top 50 users by PL points (Section 3).

Design

• Table: rank, address (truncated or Farcaster username if available), total PL, estimated share %
• Highlight current user's row (if connected and in top 50)
• If user is not in top 50, show their rank at the bottom ("Your rank: [QA-1] Smoke test — UI pages and navigation (chrome automation) #127")
• Pagination or "Show more" if needed

Data Source

GET /api/airdrop/leaderboard?address=0x...

Files

• src/components/airdrop/Leaderboard.tsx

Acceptance Criteria

• Displays top 50 by total points
• Shows rank, address/username, total PL, share %
• Current user's row highlighted
• User's own rank shown if outside top 50
• Responsive layout