SwiftUI patterns (macOS)

Docs: https://developer.apple.com/documentation/swiftui and the macOS HIG.

• Use system-adaptive colors and materials. Do not fill NavigationSplitView sidebars or root panes with opaque custom colors.
• Use WindowGroup(..., id:) for the primary launch window. A Window scene alone will not appear at launch in a menu-bar-plus-window app.
• Keep sidebars simple. Put metadata-heavy content in detail or inspector panes.
• Do not add a view model by reflex. Pick the state owner first.
• For a new app scaffold, split into files and add one script/build_and_run.sh (see build-run-debug). A single file is only for snippets under about 50 lines.