Make validation optional for saving library properties preferences

[paudelritij]

Closes #11840
Follow-up #12867

This pull request improves file directory handling by making validation optional when saving preferences, enhancing error messaging for clarity, and streamlining the user experience to allow saving even with inaccessible directories.

Steps to test

1. Launch JabRef on the first machine and navigate to the Library Properties.
2. Set a valid file directory (e.g., Library-specific) in the properties.
3. Save the changes made to the library properties.
4. Launch JabRef on the second machine, ensuring that the directory defined in step 2 is not present.
5. Go to Library Properties and make changes that do not involve overriding the default file directories.
6. Apply the changes made to the library properties.
7. When the 'Action Required' dialog appears, click "Save" This will save all changes, including those with invalid file directories for the second machine.

Mandatory checks

• I own the copyright of the code submitted and I license 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 (if change is visible to the user)
• 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.

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨