macOS app best practices

Docs: https://developer.apple.com/design/human-interface-guidelines/designing-for-macos and https://developer.apple.com/documentation/swiftui.

• Choose scenes deliberately (WindowGroup, Window, DocumentGroup, Settings, MenuBarExtra). Wrap Mac-only scenes and modifiers in #if os(macOS) in shared code.
• Wire real menu commands and keyboard shortcuts. Add system command groups only when the scene has the matching content.
• Mac Catalyst can use only Catalyst-available APIs, so AppKit is unavailable there. Exclude incompatible frameworks with platform filters.