Refactor PLib video library manager to use ReactiveUI, replacing CommunityToolkit.Mvvm. Update dependencies, enhance thumbnail caching logic, and improve UI responsiveness with reactive commands. Remove obsolete PLib.slnx file and update README.md to reflect changes.

src/PLib.Desktop/ViewModels/DisposableExtensions.cs

@@ -0,0 +1,14 @@
+using System.Reactive.Disposables;
+
+namespace PLib.Desktop.ViewModels;
+
+internal static class DisposableExtensions
+{
+    /// <summary>
+    /// Parks a subscription in the owner's bag so it dies with the owner. ReactiveUI 24 moved
+    /// its own <c>DisposeWith</c> into a namespace whose operator set collides with
+    /// System.Reactive's, so this project keeps its own two-line version instead.
+    /// </summary>
+    public static void AddTo(this IDisposable disposable, CompositeDisposable subscriptions) =>
+        subscriptions.Add(disposable);
+}