Follow-up to #14486

1. Enable AI in JabRef - https://docs.jabref.org/ai/preferences
2. Open a library with a PDF attached - e.g., https://github.com/JabRef/jabref-demonstration-libraries/tree/main/chocolate
3. Select the PDF
4. Go to tab "AI Chat"
5. See that "Export" is enabled
   
6. Expected: The button should only be enabled if there is some chat message

This issue requires a diving into JavaFX bindings.

Also implement if for "Chat with groups"