e2e: axe-core accessibility sweep across dashboards (Playwright)

[h0lybyte]

Tracker: #11583

Problem

The Argo dashboard bug fixed in PR #11578 (`

`, no `role="button"`, no keyboard handlers) would have been flagged by axe-core as a serious accessibility violation. We have none of that today.

Acceptance

• @axe-core/playwright added to the astro-kbve e2e dev deps
• Reusable helper e2e/helpers/axe.ts exporting runAxe(page) that fails the test on serious + critical violations only (warnings are noisy)
• Existing + new dashboard specs call runAxe(page) after the page is interactive
• Initial baseline allowlist for known-but-out-of-scope violations (Starlight built-ins, etc) so the rollout doesn't drown the run in red