The native title bar goes away on desktop: macOS keeps its traffic lights
centered on a 40px strip, Windows and Linux draw their own caption buttons,
and every screen builds that strip from lib/widgets/window_chrome.dart.
main.dart's inline window_manager and single-instance setup moves behind
initializeIcarusDesktopWindow and ensureIcarusSingleInstance so web and
desktop share one call site.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>