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.
- Throw ArgumentException in DockContainer.ReplaceChild if newChild is already in the container to avoid index shifting bugs.
- Add comment in DockPanel.CheckCleanup explaining the asymmetric root panel collapse behavior.
