loading property

IDEA-53: shows a spinner in place of the label/icon and blocks taps (even though onTap is non-null) while an async action this button triggered — a purchase, a save, a cloud sync — is still in flight. Deliberately keeps the button's accent color/shadow (see _enabled vs _tappable below): visually this is "busy", not "unavailable", so it shouldn't look the same as a genuinely disabled button.