chore: fix browser build plugin

[userquin]

Description

Looks like on my Windows laptop the browser build plugin is finishing before Vite finish building the UI and so there is no UI entry point in the dist folder.

With this PR we ensure the browser/dist/client/__vitest__ folder will always have at least the UI entry point.

/cc @patak-dev IIRC Vite will copy the entry point after building the assets, please confirm this (I think it is true: I deal this it in the PWA plugin)

✔️ confirmed (check step 10 here ;) ) https://vite-pwa-org.netlify.app/guide/cookbook.html#vite-build-cli

Vitest hanging when running any browser test with the UI in my local, running the test:browser:preview script with --reporter=hanging-process:

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[hi-ogawa]

Makes sense. I can reproduce empty __vitest__ by following commands locally too:

$ rm -rf packages/ui/dist/
$ pnpm --filter @vitest/browser build
...
[copy-ui-plugin] Waiting for UI to be built..

# then in another terminal
$ pnpm --filter @vitest/ui build