feat(react): announce arrivals, document client sharing

[soufian3hm]

Completes the last two deferred items from #61 (follow-up to #72, which covers the other four):

• aria-live for the new-arrival pill: a visually hidden role=status region carries the same localized text as the pill, so screen-reader users perceive arrivals held behind it without moving focus. Emptied on dismiss. Deliberately scoped to the pill: always-on badge announcements would read every unseen tick during bursts, while the badge count already rides the bell's accessible name (announced on focus, per the audit note).
• One client per page docs: sdk-reference gains a section (plus a react README pointer) on sharing one ChimelyProvider client across multiple inbox surfaces, since each standalone <Inbox /> opens its own SSE connection into the browser's ~6-per-origin HTTP/1.1 cap.

Verification

biome, build, typecheck, vitest all green: 106 react tests, including a new one pinning the status region (announces on arrival behind the pill, empties on dismiss). Patch changeset included.

[greptile-apps]

Greptile Summary

This PR adds accessible arrival announcements and client-sharing guidance. The main changes are:

• A hidden polite status region for new arrivals behind the pill.
• Shared visually hidden CSS for the status region.
• Test coverage for announcing and clearing the arrival message.
• Docs recommending one shared ChimelyProvider client per page.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
packages/react/src/components/NotificationList.tsx	Adds a hidden role="status" region that mirrors the pending new-notification pill text.
packages/react/src/styles.ts	Adds the .chimely-sr-only utility for visually hidden status text.
packages/react/src/theming.test.tsx	Adds coverage for announcing arrivals and clearing the status text on dismiss.
docs/content/docs/sdk-reference.mdx	Documents sharing one provider client across multiple inbox surfaces.
packages/react/README.md	Links React package readers to the one-client-per-page guidance.
.changeset/arrival-announcements.md	Adds a patch changeset for the React accessibility and documentation updates.

_{Reviews (1): Last reviewed commit: "feat(react): announce arrivals, document..." | Re-trigger Greptile}