Migrate styled-components to vanilla-extract

The repeatable procedure for converting this monorepo's styling from styled-components (the Studio's legacy styling library) to vanilla-extract. Adapted from the sanity-io/plugins skill of the same name for this repo's setup.

The infrastructure is already wired — never touch it during a component migration. packages/sanity and packages/@sanity/vision build .css.ts files into lib/bundle.css via vanillaExtract: true in their tsdown.config.ts, expose the ./bundle.css export, register vanillaExtractPlugin() in their vitest.config.mts, and the dev studios register it in sanity.cli.ts. A migration PR only converts component styles.

Ground rules