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

Start in `examples/web_ui` to locate the chat component files and see how it is currently exported and consumed. Then inspect the existing frontend package/build setup (notably `package.json` and related config files) to infer publishable metadata, scripts, and exports. Next, map out a standalone package layout for that component, wire its build/export config, and verify with a local package build/publish dry-run that it installs as a reusable npm package.

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