Freeze header on property mapping UI

This suggestion from a previous user, to allow access to controls at top of element/property mapping page when working down the list of properties/elements to be mapped:

<img width="734" height="413" alt="Image" src="https://github.com/user-attachments/assets/cd83c75f-29a6-4fb3-bd01-ee619251ad8f" />

This is not simple of implement and so has been taken out of issue #624 