fix(xo-web/Text): replace spaces with nbsp so that the text is always clickable

[pdonias]

See https://xcp-ng.org/forum/topic/8466

Description

On a Text field, if the user deletes the content, we show a placeholder so that the user can still see it and click it. But when the user enters white spaces, they get collapsed by the HTML rendering and the field becomes impossible to edit. This PR changes Text so that it displays non-breaking spaces instead of white spaces so the field remains visible and clickable.

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft