fix(ci): allow demo deploy to update gh-pages

[vitormattos]

Summary

Grant the demo deployment workflow write access to repository contents.

The current workflow builds the demo successfully, but the deploy step fails when JamesIves/github-pages-deploy-action@v4 attempts to push to gh-pages.

The failing run reports:

GITHUB_TOKEN Permissions
Contents: read

and then:

remote: Permission to LibreSign/pdf-elements.git denied to github-actions[bot].
fatal: unable to access 'https://github.com/LibreSign/pdf-elements.git/': The requested URL returned error: 403

Change

Add:

permissions:
  contents: write

This keeps the existing deployment approach and grants only the permission required for the workflow to update the gh-pages branch.