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.cs

@@ -18,6 +18,8 @@ public partial class CheckBoxHandler : ViewHandler<ICheckBox, SkiaCheckBox>
         [nameof(ICheckBox.IsChecked)] = MapIsChecked,
         [nameof(ICheckBox.Foreground)] = MapForeground,
         [nameof(IView.Background)] = MapBackground,
+        [nameof(IView.VerticalLayoutAlignment)] = MapVerticalLayoutAlignment,
+        [nameof(IView.HorizontalLayoutAlignment)] = MapHorizontalLayoutAlignment,
     };
 
     public static CommandMapper<ICheckBox, CheckBoxHandler> CommandMapper = new(ViewHandler.ViewCommandMapper)
@@ -83,4 +85,32 @@ public partial class CheckBoxHandler : ViewHandler<ICheckBox, SkiaCheckBox>
             handler.PlatformView.BackgroundColor = solidPaint.Color.ToSKColor();
         }
     }
+
+    public static void MapVerticalLayoutAlignment(CheckBoxHandler handler, ICheckBox checkBox)
+    {
+        if (handler.PlatformView is null) return;
+
+        handler.PlatformView.VerticalOptions = checkBox.VerticalLayoutAlignment switch
+        {
+            Primitives.LayoutAlignment.Start => LayoutOptions.Start,
+            Primitives.LayoutAlignment.Center => LayoutOptions.Center,
+            Primitives.LayoutAlignment.End => LayoutOptions.End,
+            Primitives.LayoutAlignment.Fill => LayoutOptions.Fill,
+            _ => LayoutOptions.Fill
+        };
+    }
+
+    public static void MapHorizontalLayoutAlignment(CheckBoxHandler handler, ICheckBox checkBox)
+    {
+        if (handler.PlatformView is null) return;
+
+        handler.PlatformView.HorizontalOptions = checkBox.HorizontalLayoutAlignment switch
+        {
+            Primitives.LayoutAlignment.Start => LayoutOptions.Start,
+            Primitives.LayoutAlignment.Center => LayoutOptions.Center,
+            Primitives.LayoutAlignment.End => LayoutOptions.End,
+            Primitives.LayoutAlignment.Fill => LayoutOptions.Fill,
+            _ => LayoutOptions.Start
+        };
+    }
 }