feat: add Leaderboard Display with Conference Management (#486)

[Escelit]

What

Implements the Leaderboard Display with Conference Management feature as described
in issue #486.

Changes

New files:

• src/components/leaderboard/LeaderboardConference.tsx — Core component with two
  sections:

  • Leaderboard — ranked participant list with crown/medal icons for top 3,
    formatted scores, and full ARIA accessibility
  • Conference Management — create, list, join/leave, and delete conference rooms;
    joining a room renders the existing VideoConference WebRTC component inline for live
    peer-to-peer sessions

• src/app/leaderboard/page.tsx — Next.js App Router page at /leaderboard

• src/components/leaderboard/tests/LeaderboardConference.test.tsx — 19 unit
  tests

How it was tested

• ✅ All 19 unit tests pass (pnpm vitest run)
• ✅ No TypeScript errors in new files
• ✅ No regressions in existing tests

Acceptance criteria checklist

• Leaderboard Display properly implements Conference Management
• All related tests pass
• No regression in existing functionality
• Code follows project coding standards (Tailwind CSS, lucide icons, App Router,
  dark mode)
• Accessibility guidelines followed (ARIA labels, aria-pressed, aria-expanded,
  semantic landmarks)
• Performance impact is minimal (no new dependencies added)

Notes

The build was already failing before this PR due to a pre-existing syntax error in
src/lib/api.ts — unrelated to this feature.

Closes #486

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