Adds @axe-core/playwright with a shared fixture providing an `axe`
helper. New a11y.spec.ts runs WCAG 2.1 AA checks on signin, dashboard,
timeline, allocations, resources, and projects pages. Currently reports
violations as warnings — upgrade to hard failures after fixes.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
