Accessibility regression suite: axe-core in CI and screen reader test plan

[PAMulligan]

Summary

Protect the existing WCAG 2.1 AA / Lighthouse 98 score with automated axe-core checks in CI and a documented manual screen-reader test plan for each release.

Motivation

Accessibility regresses easily and quietly. The current score is hard-won; we should lock it in rather than treating it as a one-shot achievement.

Acceptance Criteria

• @axe-core/playwright integration in the existing E2E suite checking widget closed, open, with messages, and error states
• CI fails on any new serious or critical axe violation
• Storybook + @storybook/addon-a11y for in-development feedback
• Manual test plan in docs/accessibility.md: VoiceOver (Safari), NVDA (Firefox), and TalkBack (Android Chrome) flows for sending a message, scrolling history, and dismissing the widget
• Release checklist updated with the manual a11y pass