📌 feat: Add Support for Persistable (Non-Dismissible) Banners

[berry-13]

Summary

This PR adds support for non-dismissible “persistable” banners. The schema and validation now include a persistable flag, the update script can set it, and the UI ensures these banners always appear without a dismiss button. This enables reliable, permanent system-wide announcements

Docs: LibreChat-AI/librechat.ai#459

Closes #7991

Change Type

• New feature (non-breaking change which adds functionality)

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes
• Any changes dependent on mine have been merged and published in downstream modules.