Autosave UX enhancements

[thibaudcolas]

Follow-up improvements to the Autosave MVP based on user feedback and real-world usage.

Intended outcome

• Deferred validation for required fields in StreamField #13699
• UI polish based on feedback from v7.3 release
• Improved autosave experience based on real-world usage patterns

More information

• Autosave #47
• Deferred validation for required fields in StreamField #13699
• UI polish based on feedback from v7.3 release #13863