Preview 3: Complete control implementation with XAML data binding

Major milestone adding full control functionality:

Controls Enhanced:
- Entry/Editor: Full keyboard input, cursor navigation, selection, clipboard
- CollectionView: Data binding, selection highlighting, scrolling
- CheckBox/Switch/Slider: Interactive state management
- Picker/DatePicker/TimePicker: Dropdown selection with popup overlays
- ProgressBar/ActivityIndicator: Animated progress display
- Button: Press/release visual states
- Border/Frame: Rounded corners, stroke styling
- Label: Text wrapping, alignment, decorations
- Grid/StackLayout: Margin and padding support

Features Added:
- DisplayAlert dialogs with button actions
- NavigationPage with toolbar and back navigation
- Shell with flyout menu navigation
- XAML value converters for data binding
- Margin support in all layout containers
- Popup overlay system for pickers

New Samples:
- TodoApp: Full CRUD task manager with NavigationPage
- ShellDemo: Comprehensive control showcase

Removed:
- ControlGallery (replaced by ShellDemo)
- LinuxDemo (replaced by TodoApp)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Rendering/SkiaRenderingEngine.cs

@@ -3,6 +3,7 @@
 
 using SkiaSharp;
 using Microsoft.Maui.Platform.Linux.Window;
+using Microsoft.Maui.Platform;
 using System.Runtime.InteropServices;
 
 namespace Microsoft.Maui.Platform.Linux.Rendering;
@@ -86,7 +87,13 @@ public class SkiaRenderingEngine : IDisposable
         
         // Draw popup overlays (dropdowns, calendars, etc.) on top
         SkiaView.DrawPopupOverlays(_canvas);
-        
+
+        // Draw modal dialogs on top of everything
+        if (LinuxDialogService.HasActiveDialog)
+        {
+            LinuxDialogService.DrawDialogs(_canvas, new SKRect(0, 0, Width, Height));
+        }
+
         _canvas.Flush();
 
         // Present to X11 window