[v0.9] Wire up theme in React renderer basic catalog

Implement plumbing to apply the surface theme (specifically `primaryColor`) to components in the React basic catalog. This involves setting up CSS variables at the surface level and ensuring components consume them.

This is a child issue of https://github.com/google/A2UI/issues/683.

Look at the parent issue #683 for context on the surface theme system. Find the React basic catalog components and the existing theme setup. The task is to define CSS variables for `primaryColor` at the surface level and update component styles to use these variables. Check for existing theme utilities or context providers. Verify the changes by running the catalog and inspecting the applied styles.

Written by the indexing model from the issue text.