frontend: run tests in ci in single thread

CI sometimes still hangs, trying to run without threads

- vitest-dev/vitest#2008
- vitest-dev/vitest#3077

In earlier vitest versions the optinon was --no-threads, but this
changed to --pool=forks in:
- https://github.com/vitest-dev/vitest/releases/tag/v1.0.0-beta.0
- https://github.com/vitest-dev/vitest/blob/main/docs/guide/migration.md#pools-are-standardized-4172

i# with '#' will be ignored, and an empty message aborts the commit.

scripts/ci.sh

@@ -27,7 +27,7 @@ golangci-lint run
 
 npm --prefix=frontends/web install # needed to install dev dependencies.
 make weblint
-npm --prefix=frontends/web test -- --no-color --no-watch
+npm --prefix=frontends/web test -- --no-color --no-watch --pool=forks
 # check that the i18n files are formatted correctly (avoids noisy diff when
 # pulling from locize)
 if ! locize format frontends/web/src/locales --format json --dry true ; then