fix(PostIt): disable links without a reader context

Prevent the Markdown viewer's default browser handler from activating when the blog reader has no view model, while keeping reader links bound to PostIt after context changes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/PostIt/PostIt.Desktop/PostIt.Desktop.csproj

@@ -24,4 +24,4 @@
   <ItemGroup>
     <ProjectReference Include="..\PostIt\PostIt.csproj" />
   </ItemGroup>
-</Project>
+</Project>

src/PostIt/PostIt.Tests/BlogReaderTests.cs

@@ -130,7 +130,8 @@ public class BlogReaderTests
             var page = new BlogsPage();
             var reader = page.FindControl<MarkdownScrollViewer>("ArticleReader")!;
             var engine = Assert.IsAssignableFrom<IMarkdownEngine2>(reader.Engine);
-            Assert.Null(engine.HyperlinkCommand);
+            Assert.NotNull(engine.HyperlinkCommand);
+            Assert.False(engine.HyperlinkCommand.CanExecute("https://example.test/"));
             page.DataContext = vm;
             var nav = new NavigationPage();
             await nav.PushAsync(page);
@@ -152,7 +153,8 @@ public class BlogReaderTests
             Dispatcher.UIThread.RunJobs();
             Assert.Empty(vm.ReaderTitle);
             Assert.Null(vm.ReaderDateModified);
-            Assert.Null(engine.HyperlinkCommand);
+            Assert.NotNull(engine.HyperlinkCommand);
+            Assert.False(engine.HyperlinkCommand.CanExecute("https://example.test/"));
             Assert.DoesNotContain(sink.Entries, entry => entry.Area == LogArea.Binding);
         }
         finally

src/PostIt/PostIt/Views/Blogs/BlogsPage.axaml.cs

@@ -4,20 +4,32 @@ using Avalonia.Controls;
 using Avalonia.Platform.Storage;
 using Avalonia.Controls.Primitives;
 using Avalonia.Interactivity;
+using CommunityToolkit.Mvvm.Input;
 using Markdown.Avalonia;
 
 namespace PostIt.Views.Blogs;
 
 public partial class BlogsPage : ContentPage
 {
+    private static readonly RelayCommand<string?> DisabledLinkCommand =
+        new(static _ => { }, static _ => false);
+
     public BlogsPage()
     {
         InitializeComponent();
-        DataContextChanged += (_, _) =>
+        DataContextChanged += (_, _) => UpdateHyperlinkCommand();
+        UpdateHyperlinkCommand();
+    }
+
+    private void UpdateHyperlinkCommand()
+    {
+        if (ArticleReader.Engine is IMarkdownEngine2 engine)
         {
-            if (ArticleReader.Engine is IMarkdownEngine2 engine)
-                engine.HyperlinkCommand = (DataContext as ViewModels.BlogsViewModel)?.OpenResourceCommand;
-        };
+            if (DataContext is ViewModels.BlogsViewModel vm)
+                engine.HyperlinkCommand = vm.OpenResourceCommand;
+            else
+                engine.HyperlinkCommand = DisabledLinkCommand;
+        }
     }
 
     protected override async void OnApplyTemplate(TemplateAppliedEventArgs e)