feat(ui): add DataTable design-system component

[thebabalola]

Closes #971

Summary

Adds a shared, accessible DataTable component to the design system under src/components/ui/, filling the gap where each page (leaderboards, transaction history, admin lists) was building ad-hoc table UI.

What changed

frontend/src/components/ui/DataTable.jsx

• Sortable columns — <th scope="col"> with aria-sort, real <button> controls so sort is fully keyboard-reachable
• Pagination — delegates directly to the existing <Pagination /> design-system component (no duplication)
• Three states: loading (shimmer skeleton rows, aria-busy on <tbody>), empty (configurable message), error (role="alert" on the spanning cell)
• Polite aria-live region announces the active sort to screen readers after each change
• Custom cell renderer: any column accepts a render(value, row) function
• Table carries role="grid" for correct screen-reader announcement

frontend/src/components/ui/DataTable.css

• Scoped .ds-table-* BEM classes, entirely themed through --ds-* tokens from tokens.css
• Shimmer animation for skeleton rows with prefers-reduced-motion guard
• Light-theme override via :root[data-theme='light']

frontend/src/components/ui/DataTable.test.jsx

• 18 unit tests (all passing): nextSort helper, column rendering, custom cell render, empty / loading / error states, sort click interactions, pagination delegation, and ARIA attribute checks

frontend/src/stories/DataTable.stories.jsx

• 7 Storybook stories: Default, Sortable, WithPagination (interactive state), Loading, Empty, Error, CustomCellRender, LightTheme

frontend/src/components/ui/index.js

• Exported DataTable and nextSort from the design-system barrel

Local CI verification

• npx vitest run src/components/ui/DataTable.test.jsx → 18/18 tests passed ✅
• npm run format:check → all new/modified files pass prettier ✅
• Remaining 2 prettier warnings (SECURITY.md, .github/FUNDING.yml) are pre-existing in upstream/main and unrelated to this PR

[thebabalola]

hey @joelpeace48-cell, here's my implementation for issue #971 — added the DataTable component to the design system with sorting, pagination, empty/loading/error states, 18 unit tests, and 7 Storybook stories. happy to adjust anything!

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[thebabalola]

hey @joelpeace48-cell, fixed the lint errors in the DataTable stories — the WithPagination story had a lowercase render function calling useState, which tripped the react-hooks rule. renamed it to Render and the frontend lint/build/test suite all passes locally now. let me know if anything else looks off!