Replace the inline Plain/Accent/Colorful picker in Home customization with
a navigation row that opens a dedicated page. The new page keeps the style
selector and renders a non-interactive preview of every shortcut type so
the chosen style is visible before committing.
- Add an optional styleOverride to HomeShortcutCardView so the preview can
reflect the not-yet-saved selection.
- On macOS, navigate via navigationDestination(isPresented:) instead of a
List-embedded NavigationLink, which otherwise stays stuck in its selected
state after popping back and can't be reopened.
