feat(webui): publish chat component as a standalone npm package

**Background**

The chat component currently lives inside `examples/web_ui` as part of the broader WebUI example. Making it available as a standalone npm package would allow developers to easily integrate the AgentScope chat interface into their own applications without having to copy or vendor the source directly.

**Changes**
- Extract the chat component from `examples/web_ui` into a standalone, independently publishable package
- Configure `package.json`, build setup, and exports to support npm publishing
- Publish the package to npm for public consumption