fix(ui): rebuild dist automatically on CSS Modules changes in dev

[gohabereg]

Summary

• playground's dev command builds the whole workspace and watches for changes, but the ui package never rebuilt automatically after the initial build — only a manual yarn build picked up changes.
• Root cause: ui's dev script (vite build --watch) relies on Rollup's dependency graph to decide what to rebuild. Our CSS Modules (.pcss) pipeline never registers that graph edge, so editing a .pcss file alone never triggered a rebuild. Editing a .ts file did trigger one, and it happened to pick up whatever CSS was on disk at that moment — which made it look like ui just doesn't watch, since most day-to-day work on this package is CSS.
• Fix: switch ui's dev script to chokidar-cli, which watches every file under src (any type) and reruns a full yarn build per change, sidestepping Rollup's incomplete CSS dependency tracking. Left a comment in vite.config.ts explaining why vite build --watch isn't used directly, since it's a non-obvious workaround for a caching bug in vite-plugin-css-injected-by-js (confirmed by reproducing the same one-rebuild-behind staleness across three different CSS pipelines: postcss-modules, vite-css-modules, and native Lightning CSS).

Test plan

• yarn build in packages/ui succeeds
• Ran playground's yarn dev end-to-end; edited a .pcss-only change and confirmed dist/ui.js rebuilds automatically and renders correctly in the browser on refresh, with no manual yarn build needed
• Verified no console errors in the running playground app

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