Add React 19 peer dep support

[cole-robertson]

Why

If a consumer is on React 19, then they get an error about the React 18 peer dependency constraint.

What

This PR enables React 19 peer dep support for all llm-ui subpackages.

Testing

This seems to have no issues from my testing with React 19 compatibility.

Closes #288

[vercel]

@cole-robertson is attempting to deploy a commit to the llm-ui Team on Vercel.

A member of the Team first needs to authorize it.

[rokikon]

Wouldn't it be more explicit to use ^19.0.0? Consumers still could use older versions if they need to stay on React 18.

[saliftankoano]

Yes please merge this as soon as possible. I'm literally running into this problem with React 19. Thank you 🙏

[coezbek]

In your package.json add:

  "overrides": {
    "@llm-ui/react": {
      "react": "^19.1"
    },
    "@llm-ui/markdown": {
      "react": "^19.1"
    },
    "@llm-ui/code": {
      "react": "^19.1"
    }
  }