Startup error handling

src/GuiApp/Avalonia/FileTime.GuiApp/ViewModels/MainWindowLoadingViewModel.cs

@@ -1,4 +1,5 @@
 using System.Reactive.Subjects;
+using DeclarativeProperty;
 
 namespace FileTime.GuiApp.ViewModels;
 
@@ -6,4 +7,6 @@ public class MainWindowLoadingViewModel : IMainWindowViewModelBase
 {
     public bool Loading => true;
     public IObservable<string?> MainFont { get; } = new BehaviorSubject<string?>("");
+    public DeclarativeProperty<string> Title { get; } = new("Loading...");
+    public DeclarativeProperty<string> FatalError { get; } = new();
 }