Design and build native-feeling macOS application UIs. Use this skill whenever the user asks to create a desktop app, macOS app, Mac-style interface, Apple-style UI, system utility, or anything that should look and feel like a native Mac application. Also trigger when users mention "native feel", "desktop app design", "Apple design patterns", "sidebar layout", "traffic lights", or want to build tools/utilities that feel like they belong on macOS. This skill covers layout, composition, interaction patterns, animations, light/dark mode, and all the subtle details that make an app feel like Apple built it.