feat(launcher): stabilize prepared content updates

tests/AcDream.Launcher.Tests/LauncherUpdateViewModelTests.cs

@@ -152,6 +152,27 @@ public sealed class LauncherUpdateViewModelTests
         Assert.Equal(0, updater.StageCalls);
     }
 
+    [Fact]
+    public async Task AnotherStartupQuestionDefersButDoesNotLoseTheUpdatePrompt()
+    {
+        var updater = new FakeUpdater { ClientUpdateAvailable = true };
+        bool canOpen = false;
+        using var viewModel = new LauncherUpdateViewModel(
+            updater,
+            new ImmediateUiDispatcher(),
+            () => { },
+            canOpen: () => canOpen,
+            canMutate: () => true);
+
+        await viewModel.StartupCheckAsync();
+        Assert.False(viewModel.IsOpen);
+
+        canOpen = true;
+        viewModel.TryOpenPendingUpdate();
+
+        Assert.True(viewModel.IsOpen);
+    }
+
     [Fact]
     public async Task UpdatingIsRefusedWhileASessionIsRunning()
     {