feat: add trending creator keys leaderboard

[Stanley-Owoh]

Add trending leaderboard section on homepage

New users landing on the homepage had no signal of which creator keys are popular. This PR adds a leaderboard section fetching from GET /leaderboard/volume and rendering the top 20 keys with rank, name, volume, and 24h price change.

Changes

New files:

• src/services/leaderboard.service.ts — API service for GET /leaderboard/volume
• src/hooks/useLeaderboardVolume.ts — React Query hook with 5-minute stale time
• src/components/home/TrendingLeaderboard.tsx — Homepage leaderboard section with skeleton loader
• src/pages/LeaderboardPage.tsx — Dedicated leaderboard page

Modified files:

• src/lib/queryKeys.ts — Added leaderboard.volume() query key
• src/routes.tsx — Added /leaderboard route
• src/pages/HomePage.tsx — Integrated <TrendingLeaderboard /> between Spotlight and TrendingCreators

Acceptance Criteria

• 20 rows rendered with correct rank, name, volume, and price change
• Positive price change displayed in green, negative in red
• Skeleton shown during initial load
• Data refreshes automatically after 5 minutes
• 'View all' link navigates to leaderboard page

Closes #779

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