Fix macOS update not working.

Rename `MacOS.Update` namespace and folder to `MacOS.PlatformUpdate` and update references accordingly.

src/PicView.Avalonia.MacOS/Update/MacUpdateHelper.cs → src/PicView.Avalonia.MacOS/PlatformUpdate/MacUpdateHelper.cs

@@ -3,7 +3,7 @@ using PicView.Avalonia.Update;
 using PicView.Avalonia.WindowBehavior;
 using PicView.Core.DebugTools;
 
-namespace PicView.Avalonia.MacOS.Update;
+namespace PicView.Avalonia.MacOS.PlatformUpdate;
 
 /// <summary>
 ///     Handles macOS-specific update logic

src/PicView.Avalonia.MacOS/Views/AboutWindow.axaml.cs

@@ -2,7 +2,7 @@ using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Media;
 using PicView.Avalonia.Interfaces;
-using PicView.Avalonia.MacOS.Update;
+using PicView.Avalonia.MacOS.PlatformUpdate;
 using PicView.Avalonia.UI;
 using PicView.Avalonia.Update;
 
@@ -13,6 +13,7 @@ public partial class AboutWindow : Window, IPlatformSpecificUpdate
     public AboutWindow()
     {
         InitializeComponent();
+        XAboutView.PlatformUpdate = this;
         if (!Settings.Theme.Dark || Settings.Theme.GlassTheme)
         {
             XAboutView.Background = Brushes.Transparent;

src/PicView.Avalonia.MacOS/WindowImpl/WindowHelper.cs

@@ -4,7 +4,7 @@ using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Threading;
 using PicView.Avalonia.Functions;
 using PicView.Avalonia.Interfaces;
-using PicView.Avalonia.MacOS.Update;
+using PicView.Avalonia.MacOS.PlatformUpdate;
 using PicView.Avalonia.MacOS.Views;
 using PicView.Avalonia.Update;
 using PicView.Avalonia.ViewModels;