Feat/611 universal data grid

[ThatCodeBabe]

Summary

closes #611

• Built on @tanstack/react-table + @tanstack/react-virtual for sorting, filtering, column pinning, and row virtualization
• Row virtualization renders only visible rows, tested against a 5,000-row synthetic dataset
• Per-column text filtering (substring match, debounced)
• Click-to-sort headers (asc → desc → none)
• Column pinning left/right via a per-header pin menu, with sticky positioning
• Export CSV button exports the currently filtered + sorted rowset
• Full WAI-ARIA grid keyboard navigation: arrow keys move focus between cells, Enter/Space sorts a focused header column, Tab reaches toolbar and filter inputs
• Wired into app/demo/page.tsx alongside existing demo components

Acceptance criteria

• Virtualization
• Sorting
• Filtering
• Column pinning
• Export
• Keyboard accessibility

Test plan

• tsc --noEmit — no errors in new/modified files
• eslint — no errors or warnings in new/modified files
• Manually verified in a running dev server (Playwright-driven): sorted a column, filtered Ticket Type down to 22 VIP rows out of 5,000, pinned the Email column left, arrow-key + Enter keyboard navigation, triggered a real CSV download, no console errors