feat: add themed error boundary fallback for LeaderboardTable (Closes #813)

[waterWang]

Summary

Adds a polished, leaderboard-themed error boundary fallback for the LeaderboardTable component. When a render error is caught, users see a friendly message with a Retry button instead of a blank screen or the generic ErrorRecoveryScreen.

Changes

New files

• components/leaderboard/LeaderboardTableErrorFallback.tsx — Custom fallback UI that matches the dark-slate leaderboard palette. Includes:

  • Trophy icon & "Leaderboard temporarily unavailable" heading
  • Explanatory text
  • Incident ID display (development only)
  • Retry button (cyan accent, matching table theme)
  • Collapsible error details (development only)

• components/leaderboard/LeaderboardTableErrorBoundary.tsx — Class-based error boundary that wraps render content and catches errors. Exports:

  • LeaderboardTableErrorBoundary — reusable boundary class
  • LeaderboardTableWithBoundary — convenience wrapper around the LeaderboardTable
  • LeaderboardTable re-export for backward compatibility

• components/leaderboard/__tests__/LeaderboardTableErrorBoundary.test.tsx — 6 tests covering:

  • Normal render (no error)
  • Fallback render when child throws
  • Retry button interaction
  • Heading and button existence in standalone fallback
  • Incident ID visibility in development
  • Smoke test for LeaderboardTableWithBoundary

Testing

• All 6 new tests pass
• Pre-existing test suite shows identical results (153 pre-existing failures, unchanged by these changes)

Closes #813

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