Dismiss sheets when navigation arrives from outside the app

Tapping a new-videos notification while Home's Settings sheet was up
switched to the Subscriptions feed underneath the sheet, so the app
came back still showing Settings. The existing dismissSettings() hook
was never called.

NavigationCoordinator.dismissPresentations() bumps a generation that
the main window's root sheet host and screen sheets (Home's Settings,
Remote Devices, Customize Home; Subscriptions' view options and data)
observe via .dismissesOnExternalNavigation(_:). The notification,
deep links and Handoff call it; in-app navigation and layout swaps do
not. The expanded player keeps its own sheets.

The notification also collapses the expanded player so the feed it
opens is visible.

Yattee/ContentView.swift

@@ -26,7 +26,10 @@ struct ContentView: View {
         }
         // Shared sheets for the main window. The expanded player hosts its own,
         // and takes global requests while it is up.
-        .appSheetHost(isTopLevel: appEnvironment?.navigationCoordinator.isPlayerExpanded != true)
+        .appSheetHost(
+            isTopLevel: appEnvironment?.navigationCoordinator.isPlayerExpanded != true,
+            dismissesOnExternalNavigation: true
+        )
         .toastOverlay()
     }