fix(workspace): release the session when the workspace view cannot be built

If the first board read failed while WorkspaceViewModel was being constructed,
the open-failed page appeared but the lock and connection stayed held until
exit, which blocked other instances. The session is now closed before the
error page is shown.

YKanBan/Services/AppServices.cs

@@ -63,6 +63,16 @@ public sealed class AppServices : IDisposable
         return Session;
     }
 
+    /// <summary>
+    /// Releases the open workspace session, if any, so its lock and connection
+    /// do not outlive a workspace that could not be shown.
+    /// </summary>
+    public void CloseWorkspace()
+    {
+        Session?.Dispose();
+        Session = null;
+    }
+
     /// <summary>
     /// Persists the configuration when this session changed a setting; called
     /// once at shutdown. A write failure only loses this session's settings

YKanBan/ViewModels/MainWindowViewModel.cs

@@ -114,7 +114,17 @@ public sealed partial class MainWindowViewModel : ViewModelBase
     /// <param name="session">The opened workspace session.</param>
     private void ShowWorkspace(string folderPath, WorkspaceSession session)
     {
-        Content = new WorkspaceViewModel(session, ShowSettings, ShowAbout, _services.Config, _dialogs);
+        try
+        {
+            Content = new WorkspaceViewModel(session, ShowSettings, ShowAbout, _services.Config, _dialogs);
+        }
+        catch
+        {
+            // The first board read failed: the caller shows an error page, which holds no workspace.
+            _services.CloseWorkspace();
+            throw;
+        }
+
         Title = $"{new DirectoryInfo(folderPath).Name} - {Resources.App_Name}";
     }