Added Vitest to web/, ready for tests

web/README.md

@@ -18,8 +18,9 @@ prefix for production builds.
 then copies `dist/` to `../internal/frontend/dist/`, where the backend embeds
 it: the next Go build of `job-search` serves it. The copy isn't committed.
 
-Before committing, `npm run check` type-checks, lints (ESLint) and checks
-formatting (Prettier); `npm run format` formats everything. See
+Before committing, `npm run check` type-checks, lints (ESLint), checks
+formatting (Prettier) and runs the tests (Vitest); `npm run format` formats
+everything, and `npm run test:watch` reruns the tests as files change. See
 [CODE_CONVENTIONS.md](CODE_CONVENTIONS.md).
 
 The backend's types in `src/types/api.ts` are generated from its OpenAPI