Liquid Glass (macOS)

Docs: https://developer.apple.com/documentation/swiftui/applying-liquid-glass-to-custom-views. The API is new, so check current docs.

• Let system sidebars, toolbars, sheets, and controls provide glass. Do not rebuild them. Add custom glassEffect only for app-specific surfaces.
• No opaque custom backgrounds behind NavigationSplitView. Keep related glass in one container.
• Tint only when color carries meaning.
• Do not port iPhone tab bar or search behavior to the Mac.
• A GUI SwiftPM app must be launched as an .app bundle (see build-run-debug).