Hide tab bar

[priyanshu16095]

Fixes #9971

This PR introduces a feature to hide the tab bar when only a single library is open in JabRef, enhancing the user interface for devices with low screen space.

THE CHANGES MADE

• When a single library is open, the tab bar is hidden.
• Ensured that the tab bar is restored when more than one library is open, maintaining expected functionality.

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions]

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[Siedlerchr]

you can just commit and push your fixes to the branch, PR will be updated automatically

[priyanshu16095]

It is also showing the changes made in the previous PR, and the tests are also failing.