chore(frontend): OpenHands design library scaffold

[amanape]

We're setting up the foundation for a shared component library that will be used across OpenHands and potentially other internal or external projects.

This library will house reusable, accessible, and styled React components, along with design tokens, themes, and utilities.

---

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:6ef49b4-nikolaik   --name openhands-app-6ef49b4   docker.all-hands.dev/all-hands-ai/openhands:6ef49b4

[amanape]

Is this component library standalone enough that it can go to its own repo?

Yes preferably, but after short discussion with @rbren, we'll keep it here for now.

empty folder with openhands-ui could be a little confusing to people i think..

Yes I sort of agree as well though it won't be empty for long. I couldn't come up with better names 😅