Add optional message to activity view

[wegenerlabs]

• Add support for a message below the spinner.
• Refactor createToastActivityView to use a stack view instead of hardcoded frames.
• Drop activitySize in ToastStyle in favor of padding constants.
  • When no message is set, use a padding of 32 pts. The .whiteLarge activity view has a width of ~37 pts, so that gives us about the same total width as before (~100 pts).
  • When a message is set, use a padding of 16 pts. It looks better if the padding is more compact in that case.

Thank you for considering my PR! Hope we can merge this - or something like it!

Screen.Recording.2025-08-14.at.15.58.31.mov