UI: Make `Dialog.Header` and `Dialog.Footer` sticky

[jameskoster]

Here's how scrolling Dialogs behave currently:

Screen.Recording.2026-04-09.at.09.19.33.mov

For parity with Modal and as a general UX enhancement I think we ought to make the header sticky.

It should be possible for Dialog.Footer to be sticky as well, but I'm unsure if that should be optional or hard-coded.