Files section: use upload i18n string for add button

[bezin]

Description

Even though the underlying action is the same, the files field and files section use different labels for the upload action: The field uses upload whereas the section uses add).

Expected behavior
I would expect both to share the same label (upload, in that case)

Screenshots

Your setup

Kirby 4.6.1

[distantnative]

Note for future fix:
Currently all models section use the add i18n string and only allow customizing the icon for that button.

We should refactor the buttons computed prop, break it up into smaller parts and make them easier to customize (e.g. having separate addButton etc. computed props).