Add @axe-core/playwright with Playwright test runner that asserts zero
WCAG 2A/2AA violations across index, live, fleet, setup, and
integrations pages. Fix contrast violations on integrations page: use
darker blue for primary buttons, lighter text for descriptions/hints,
and add body-level dark background in layout.css.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
