fix(apple): enforce a single window on macOS and iPadOS

macOS uses a single-instance `Window` scene instead of `WindowGroup`, which
otherwise lets the app open multiple windows (via ⌘N). iPadOS sets
`UIApplicationSupportsMultipleScenes = false` to block a second scene via Stage
Manager / split view. (`LSMultipleInstancesProhibited` only blocks a second
process, not a second window.)

apple/VniDrop/App/VniDropApp.swift

@@ -1,5 +1,8 @@
 import SwiftUI
 
+/// Scene identifier for the single main window.
+private let mainWindowId = "main"
+
 /// Native app entry point for iOS, iPadOS, and macOS.
 /// Opens `.vnd` invitations via `onOpenURL` and routes them to the receive flow.
 @main
@@ -7,11 +10,21 @@ struct VniDropApp: App {
 	@StateObject private var externalInvitations = ExternalInvitationController()
 
 	var body: some Scene {
-		WindowGroup {
+		#if os(macOS)
+		// A single-instance `Window` (not `WindowGroup`): the app must never open a
+		// second window. `Window` also drops the ⌘N "New Window" command.
+		Window(Text(verbatim: "VniDrop"), id: mainWindowId) {
 			RootView(dependencies: makeAppDependencies(externalInvitations: externalInvitations))
 				.ignoresSafeArea()
 				.onOpenURL(perform: openInvitation)
 		}
+		#else
+		WindowGroup(id: mainWindowId) {
+			RootView(dependencies: makeAppDependencies(externalInvitations: externalInvitations))
+				.ignoresSafeArea()
+				.onOpenURL(perform: openInvitation)
+		}
+		#endif
 	}
 
 	/// Reads a `.vnd` invitation document under a security scope, enforcing the