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/BoxViewHandler.cs

@@ -0,0 +1,67 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Handlers;
+using SkiaSharp;
+
+namespace Microsoft.Maui.Platform.Linux.Handlers;
+
+/// <summary>
+/// Handler for BoxView on Linux.
+/// </summary>
+public partial class BoxViewHandler : ViewHandler<BoxView, SkiaBoxView>
+{
+    public static IPropertyMapper<BoxView, BoxViewHandler> Mapper =
+        new PropertyMapper<BoxView, BoxViewHandler>(ViewMapper)
+        {
+            [nameof(BoxView.Color)] = MapColor,
+            [nameof(BoxView.CornerRadius)] = MapCornerRadius,
+            [nameof(IView.Background)] = MapBackground,
+            ["BackgroundColor"] = MapBackgroundColor,
+        };
+
+    public BoxViewHandler() : base(Mapper)
+    {
+    }
+
+    protected override SkiaBoxView CreatePlatformView()
+    {
+        return new SkiaBoxView();
+    }
+
+    public static void MapColor(BoxViewHandler handler, BoxView boxView)
+    {
+        if (boxView.Color != null)
+        {
+            handler.PlatformView.Color = new SKColor(
+                (byte)(boxView.Color.Red * 255),
+                (byte)(boxView.Color.Green * 255),
+                (byte)(boxView.Color.Blue * 255),
+                (byte)(boxView.Color.Alpha * 255));
+        }
+    }
+
+    public static void MapCornerRadius(BoxViewHandler handler, BoxView boxView)
+    {
+        handler.PlatformView.CornerRadius = (float)boxView.CornerRadius.TopLeft;
+    }
+
+    public static void MapBackground(BoxViewHandler handler, BoxView boxView)
+    {
+        if (boxView.Background is SolidColorBrush solidBrush && solidBrush.Color != null)
+        {
+            handler.PlatformView.BackgroundColor = solidBrush.Color.ToSKColor();
+            handler.PlatformView.Invalidate();
+        }
+    }
+
+    public static void MapBackgroundColor(BoxViewHandler handler, BoxView boxView)
+    {
+        if (boxView.BackgroundColor != null)
+        {
+            handler.PlatformView.BackgroundColor = boxView.BackgroundColor.ToSKColor();
+            handler.PlatformView.Invalidate();
+        }
+    }
+}