Remove unused @vitest/browser-playwright scaffolding

vitest.browser.config.ts was dead scaffolding from #734: the provider is in no
package.json (only a transitive optional peer of vitest), no *.browser.tsx test
files exist, and no script runs the browser config. Its only effect was a
TS2307 frontend-typecheck error because tsconfig included the file but the
import couldn't resolve. Delete the config and drop it from tsconfig include.

Author: backnotprop