feat(track-b): add isolated UI lab with automated a11y checks

[Ibinola]

Description

This PR introduces a new isolated testing environment (@ourkairos/ui-lab) within the track-b sandbox to ensure our base UI components meet accessibility standards before being integrated into the main application.

Closes #292

Technical Changes

• New Package: Added @ourkairos/ui-lab to the Track-B workspace.
• Sample Components: Implemented base structures for Modal, Dropdown, Tabs, Form, and Toast.
• A11y Validations: Integrated axe-core with Vitest to automatically test components for ARIA compliance.
• Keyboard Interactions: Added unit tests using @testing-library/user-event to verify focus-trap behavior in the Modal and arrow-key navigation in Tabs.
• Audit Reporting: Added a setup script that aggregates axe-core results and automatically generates a markdown summary (a11y-audit-report.md).

Acceptance Criteria Met

• Contributors can add components and run a11y checks in isolation.
• No dependency on OG app routes.
• Includes focus-trap behavior test for modal.
• Includes automated accessibility checks and ARIA validation.
• Generates a simple audit summary report in markdown.

How to Test

1. Pull down this branch.
2. Run pnpm install from the root workspace.
3. Run pnpm --filter @ourkairos/ui-lab test.
4. Verify all tests pass, including the focus-trap and axe-core checks.
5. Check experiments/track-b/ui-lab/a11y-audit-report.md to ensure the markdown audit summary was generated correctly.