Upload frontend test coverage on failure in CI

[ejfine]

Is there an existing issue for this?

• I have searched the existing issues

Feature description

      - name: Upload test coverage on failure
        if: ${{ failure() }}
        uses: actions/upload-artifact@v4.6.2
        with:
          name: frontend-test-coverage-${{ matrix.os }}
          path: frontend/.coverage
          if-no-files-found: error```

[ejfine]

#61