feat: Virtualized lists for large campaign/participant/leaderboard sets

[joelpeace48-cell]

Background & current state

Lists (campaigns, leaderboard CampaignLeaderboard.jsx, participants) render all rows. At thousands of rows this will jank and bloat the DOM. No virtualization is used.

Goal

Virtualize large lists (campaign list, leaderboard, participants) so large datasets scroll smoothly.

Technical design

• Adopt a virtualization lib (e.g., @tanstack/react-virtual) for the heavy lists; windowed rendering + (optionally) infinite scroll over cursor pagination (NEW-027/Pagination.tsx).
• Preserve keyboard nav + accessibility (Frontend: Add keyboard navigation and WCAG AA accessibility audit #353).

Edge cases

• Variable row heights → measured/estimated sizes.
• Sticky headers + virtualization.
• Accessibility (screen readers) with windowed rows.
• Combined with filters/sort (NEW-110).

Task breakdown

• Virtualize campaign list, leaderboard, participants.
• Infinite scroll over cursor pagination.
• A11y preserved.
• Perf profile in test (10k rows @ 60fps).

Acceptance criteria

• 10k rows scroll at ~60fps without DOM bloat.

Testing & verification

• Perf profiling test; a11y check.

Out of scope

• Server pagination (exists).

Dependencies / related

• Uses NEW-027/Pagination; respects Frontend: Add keyboard navigation and WCAG AA accessibility audit #353.

Difficulty: medium · Effort: M · performance