Use case

CupertinoRadio is missing the following features:

• Fill color of an unselected radio is a linear gradient that goes from darker at the top to lighter at the bottom in dark mode

Native dark mode	Flutter dark mode

• In light mode, radio darkens when pressed
• In dark mode, radio lightens when pressed
• Default blue color of a selected radio is darker in dark mode
• Has different colors in disabled vs. enabled states

Native macOS	Flutter

CupertinoRadio is also missing tests to check that:

• It is focusable and has correct focus colors
• It uses correct default active/inactive/fill colors

Proposal

Update CupertinoRadio to latest macOS 14 visuals, and add missing tests.