Navbar does not warn when a page is not published

**Describe the bug**
As Bug #96 it would be good if CodeRed Admin would show a warning or a message when a page added to a link in the navbar has not been published yet.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'Fragments->Navigation bar' and edit your nav bar.
2. add a link to a page that is not published.
3. There is no hint that the link is not going to be shown.

**Expected behavior**
It would be good to have a hint or warning there.

**Additional context**
Add any other context about the problem here.

Start by reproducing the issue through Fragments → Navigation bar: add an unpublished page to the navbar and confirm that no hint appears. Trace the navigation-bar editing entry point and the page publication state, then add a visible warning for unpublished linked pages and verify the warning disappears for published pages.

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