a11y: ARIA, focus trap, keyboard nav, WCAG 2.1 AA (#10)

[PAMulligan]

Summary

Brings the Claudius chat widget to WCAG 2.1 AA compliance. Closes #10.

• Dialog semantics — ChatWindow is now role="dialog" with aria-labelledby pointing at the header <h2>. aria-modal="true" is set only on mobile, where the scrim actually blocks the background (honest ARIA claim).
• Keyboard control — Escape closes the widget (ignored during IME composition). Enter submits (pre-existing). Tab/Shift+Tab cycle via a new useFocusTrap hook. Input autofocuses on open; focus returns to the toggle button on close (pre-existing).
• Screen-reader announcements — Replaced the noisy role="log" aria-live="polite" container with a dedicated sr-only aria-live="polite" aria-atomic="true" region that announces only the latest assistant message. Markdown markers and long URLs are stripped for SR clarity via a new stripAnnouncementFormatting helper.
• Contrast — Audited the Tailwind palette; bumped header subtitle from text-white/90 (4.49:1, fails) to text-white (5.17:1).
• Lint enforcement — Added eslint-plugin-jsx-a11y with recommended rules plus 5 escalated to error. aria-role is configured with ignoreNonDOM: true to handle the ChatMessage component's domain role prop without line-level disables.

Test count: 135 → 154. Lint: 0 errors. Lighthouse accessibility: 97/100 (the sole flag is landmark-one-main on the Vite dev harness index.html, not on the widget component itself).

Test plan

• cd widget && pnpm install && pnpm test — all 154 tests pass
• cd widget && pnpm lint — 0 errors
• cd widget && pnpm dev then at http://localhost:5173:
  • Tab reaches the toggle button with a visible focus ring
  • Enter opens the widget; focus lands on the message input
  • Tab cycles input → send → close → input (stays inside dialog)
  • Shift+Tab wraps backward the same way
  • Escape closes the widget; focus returns to the toggle button
• NVDA / VoiceOver spot-check (human-only):
  • Opening announces "Chat, dialog"
  • Sending a message — assistant reply is announced once, politely
  • No redundant announcements from the log region (silent by design; only the dedicated live region speaks)

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