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>

Handlers/CheckBoxHandler.Linux.cs

@@ -19,6 +19,8 @@ public partial class CheckBoxHandler : ViewHandler<ICheckBox, SkiaCheckBox>
         [nameof(ICheckBox.IsChecked)] = MapIsChecked,
         [nameof(ICheckBox.Foreground)] = MapForeground,
         [nameof(IView.IsEnabled)] = MapIsEnabled,
+        [nameof(IView.Background)] = MapBackground,
+        ["BackgroundColor"] = MapBackgroundColor,
     };
 
     /// <summary>
@@ -90,4 +92,22 @@ public partial class CheckBoxHandler : ViewHandler<ICheckBox, SkiaCheckBox>
         handler.PlatformView.IsEnabled = checkBox.IsEnabled;
         handler.PlatformView.Invalidate();
     }
+
+    public static void MapBackground(CheckBoxHandler handler, ICheckBox checkBox)
+    {
+        if (checkBox.Background is SolidColorBrush solidBrush && solidBrush.Color != null)
+        {
+            handler.PlatformView.BackgroundColor = solidBrush.Color.ToSKColor();
+            handler.PlatformView.Invalidate();
+        }
+    }
+
+    public static void MapBackgroundColor(CheckBoxHandler handler, ICheckBox checkBox)
+    {
+        if (checkBox is Microsoft.Maui.Controls.VisualElement ve && ve.BackgroundColor != null)
+        {
+            handler.PlatformView.BackgroundColor = ve.BackgroundColor.ToSKColor();
+            handler.PlatformView.Invalidate();
+        }
+    }
 }