Cut react-ui 0.4.0 #111

Merge the new chat family into main and publish the stable 0.4.0 release, with notes listing every removed export and what replaced it.

Part of #94.

Starts after #110 has merged and downstream consumers have migrated to the new family on a canary (canary validation).

Why

0.4.0 removes the old chat family, so it is a breaking release. It is cut last, once the new components have been proven by consumers on a canary prerelease, so the stable version carries a family that already works in real use.

What

• Merge the feature branch feat/conversation into main.

• On a clean, up-to-date main, follow the manual release in CONTRIBUTING.md:

  npm version minor -m "chore(release): %s"
  git push --follow-tags
  gh release create "v$(node -p 'require("./package.json").version')" --generate-notes
  npm publish

• Release notes: the README's "Upgrading from 0.3" section (Remove the old chat components #110), listing every removed export and its replacement, is copied into or linked from the GitHub release.

Requirements

• R44: the release that carries the cutover is 0.4.0, cut by the publisher after the cutover has merged and consumers have migrated; its notes list every removed export and what replaced it.

Constraints

• The publisher decides the timing and states the rationale when cutting the release.
• A minor bump: the package is pre-1.0, so the minor version carries the removal.
• Only after canary validation: downstream consumers run on the new family and their checks pass.

Done when

• bun run check, bun run build and bun run test:e2e pass on the tagged commit.
• 0.4.0 is on npm and has a GitHub release.
• The release notes list every removed export and what replaced it.