feat(docking): improve tab management and error handling

Refactored DockGroup to only remove obsolete TabViewItems and restore tab selection more reliably. Updated DockGroup.xaml to enable tab reordering and add-tab button. Switched to CommunityToolkit.WinUI.Controls for GridSplitter and added a style for it. Made DockPanel, DockRegionHighlight, and DockingLayout partial classes. In App.xaml.cs, wrapped initialization in a try-catch to exit on error, and ensured process exit on window close. Improved ProjectBrowser scrollbar behavior and layout settings.

src/Editor/Ghost.Editor/View/Controls/Docking/DockingLayout.cs

@@ -8,7 +8,7 @@ namespace Ghost.Editor.View.Controls.Docking;
 /// </summary>
 [TemplatePart(Name = PART_OVERLAY_CANVAS, Type = typeof(Canvas))]
 [TemplatePart(Name = PART_HIGHLIGHT, Type = typeof(DockRegionHighlight))]
-public class DockingLayout : Control
+public partial class DockingLayout : Control
 {
     private const string PART_OVERLAY_CANVAS = "PART_OverlayCanvas";
     private const string PART_HIGHLIGHT = "PART_Highlight";