Back closes what is open

On a phone the Back button left the page under an open menu, sheet or dialog. An open overlay now says so (new
IDOverlayHost, cascaded by MainLayout; OverlayService keeps them newest last): DModal and DDropdown through the
cascade, so the D components stay free of app services, and the nav menu, PhoneSheet and the full-screen globe through
the service. MainLayout's location-changing handler lets Back close the newest overlay instead of leaving the page; a
link followed or the app's own navigation (signing out after its question, say) closes them all and goes on. Blazor
can't tell Back from the app's navigation, so a capture-phase popstate listener in main.js marks Back
(AppStatusService.TookBack). No history entries are pushed. Phone flow: Back closes the menu, a card's More sheet, the
composer and the full-screen globe one after another without leaving the feed, and with nothing open goes back.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Shared/MainLayout.razor

@@ -1,15 +1,47 @@
-@inherits LayoutComponentBase
+@inherits LayoutComponentBase
+@implements IDisposable
 
-<div class="relative flex flex-col md:flex-row w-full h-full">
-	<NavMenu />
+@* The page and its menu. The overlays it holds (dialogs, menus, sheets: IDOverlayHost, cascaded) close with the phone's
+   Back button: while one is open, Back closes the newest instead of leaving the page; a link followed or the app's own
+   navigation closes them all and goes on. *@
+<CascadingValue Value="(IDOverlayHost)Overlays" IsFixed="true">
+	<div class="relative flex flex-col md:flex-row w-full h-full">
+		<NavMenu />
 
-	<div class="block flex-1 relative w-full h-full">
-		<div class="block absolute overflow-y-auto w-full h-full p-3 md:py-4 md:pl-2 md:pr-4">
-			@Body
+		<div class="block flex-1 relative w-full h-full">
+			<div class="block absolute overflow-y-auto w-full h-full p-3 md:py-4 md:pl-2 md:pr-4">
+				@Body
+			</div>
 		</div>
 	</div>
-</div>
 
-<ToastHost />
-<PublicNoticeHost />
-<ConfirmHost />
+	<ToastHost />
+	<PublicNoticeHost />
+	<ConfirmHost />
+</CascadingValue>
+
+@code {
+	[Inject] OverlayService Overlays { get; set; }
+	[Inject] NavigationManager Navigation { get; set; }
+	[Inject] AppStatusService Status { get; set; }
+
+	IDisposable changing;
+
+	protected override void OnInitialized() => changing = Navigation.RegisterLocationChangingHandler(OnLocationChanging);
+
+	async ValueTask OnLocationChanging(LocationChangingContext context)
+	{
+		var back = !context.IsNavigationIntercepted && Status.TookBack();
+		if (!Overlays.HasOpen)
+			return;
+		if (back)
+		{
+			context.PreventNavigation();
+			await Overlays.CloseTop(context.CancellationToken);
+			return;
+		}
+		await Overlays.CloseAll(context.CancellationToken);
+	}
+
+	public void Dispose() => changing?.Dispose();
+}