Add Accessibility Regression Checklist

[edehvictor]

Summary

Add an accessibility checklist for contributors changing UI components.

Scope

• Cover keyboard access, focus states, labels, color contrast, and reduced motion.
• Link to relevant testing commands or tools.
• Keep the checklist practical for pull requests.

Acceptance Criteria

• UI contributors have a clear accessibility review checklist.
• The checklist can be referenced from PR review comments.