feat(leaderboard): handle empty and partial states

[Tekh134]

Summary

Handle leaderboard empty and partial states with production-ready loading, error, retry, stale, and permission state management.

Changes

• Add LeaderboardLoadingState, LeaderboardErrorState, LeaderboardEmptyState, LeaderboardSkeleton components
• Create useLeaderboard hook for managing loading, error, empty, stale, and retry states
• Update LeaderboardSection to use the hook and handle all states with fallback data
• Update LeaderboardTable to accept isLoading, error, onRetry props and render appropriate states
• Fix sorting bugs (sortDescending undefined, scrollParentRef typo)
• Update LeaderboardPodium to render empty state when opThree is empty
• Update LeaderboardCards to handle loading, error, and empty states
• Update YourRankChip to gracefully handle missing user data (user is optional)
• Add comprehensive tests for all states and components

Compatibility

• All existing props remain optional
• Components default to current behavior when new state props aren't provided
• No breaking changes to public interfaces

Security & Failure-Mode Handling

• Error messages are sanitized to prevent sensitive data exposure
• Retries are bounded with maxRetries to prevent infinite loops
• Unmount-safe state updates prevent memory leaks
• Stale data detection alerts the UI without blocking interaction

Test Coverage

• hooks/tests/useLeaderboard.test.ts - Hook state machine tests
• components/leaderboard/tests/leaderboard-states.test.ts - State component tests
• components/leaderboard/tests/LeaderboardTable.test.ts - Table state and sort tests
• components/leaderboard/tests/LeaderboardCards.test.ts - Cards empty/loading/error tests
• components/leaderboard/tests/LeaderboardSection.test.ts - Section integration tests
• components/leaderboard/tests/YourRankChip.test.ts - Chip missing user tests

Closes #898

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