GitLab CI artifact downloads wrap all paths in an outer zip.
When the plugin zip was inside dist/, users downloading via the
'Download artifacts' button got artifact.zip → dist/wp-dataverse-widget.zip,
which WordPress rejected ('Aucune extension valide trouvée').

Changes:
- CI now copies the final zip to the workspace root so the artifact
  bundle contains wp-dataverse-widget.zip at top level
- Add npm run build step to CI (build/ was committed but explicit
  compilation is more robust and correct)
- Update README: warn against 'Download artifacts' button, instruct
  to use 'Browse > download file' instead
- Fix license mention in README (GPL-3.0-or-later, not CeCILL-B)