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/FrameHandler.cs

@@ -0,0 +1,104 @@
+// 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 Frame on Linux using SkiaFrame.
+/// </summary>
+public partial class FrameHandler : ViewHandler<Frame, SkiaFrame>
+{
+    public static IPropertyMapper<Frame, FrameHandler> Mapper =
+        new PropertyMapper<Frame, FrameHandler>(ViewMapper)
+        {
+            [nameof(Frame.BorderColor)] = MapBorderColor,
+            [nameof(Frame.CornerRadius)] = MapCornerRadius,
+            [nameof(Frame.HasShadow)] = MapHasShadow,
+            [nameof(Frame.BackgroundColor)] = MapBackgroundColor,
+            [nameof(Frame.Padding)] = MapPadding,
+            [nameof(Frame.Content)] = MapContent,
+        };
+
+    public FrameHandler() : base(Mapper)
+    {
+    }
+
+    public FrameHandler(IPropertyMapper? mapper)
+        : base(mapper ?? Mapper)
+    {
+    }
+
+    protected override SkiaFrame CreatePlatformView()
+    {
+        return new SkiaFrame();
+    }
+
+    public static void MapBorderColor(FrameHandler handler, Frame frame)
+    {
+        if (frame.BorderColor != null)
+        {
+            handler.PlatformView.Stroke = new SKColor(
+                (byte)(frame.BorderColor.Red * 255),
+                (byte)(frame.BorderColor.Green * 255),
+                (byte)(frame.BorderColor.Blue * 255),
+                (byte)(frame.BorderColor.Alpha * 255));
+        }
+    }
+
+    public static void MapCornerRadius(FrameHandler handler, Frame frame)
+    {
+        handler.PlatformView.CornerRadius = frame.CornerRadius;
+    }
+
+    public static void MapHasShadow(FrameHandler handler, Frame frame)
+    {
+        handler.PlatformView.HasShadow = frame.HasShadow;
+    }
+
+    public static void MapBackgroundColor(FrameHandler handler, Frame frame)
+    {
+        if (frame.BackgroundColor != null)
+        {
+            handler.PlatformView.BackgroundColor = new SKColor(
+                (byte)(frame.BackgroundColor.Red * 255),
+                (byte)(frame.BackgroundColor.Green * 255),
+                (byte)(frame.BackgroundColor.Blue * 255),
+                (byte)(frame.BackgroundColor.Alpha * 255));
+        }
+    }
+
+    public static void MapPadding(FrameHandler handler, Frame frame)
+    {
+        handler.PlatformView.SetPadding(
+            (float)frame.Padding.Left,
+            (float)frame.Padding.Top,
+            (float)frame.Padding.Right,
+            (float)frame.Padding.Bottom);
+    }
+
+    public static void MapContent(FrameHandler handler, Frame frame)
+    {
+        if (handler.PlatformView is null || handler.MauiContext is null) return;
+
+        handler.PlatformView.ClearChildren();
+
+        var content = frame.Content;
+        if (content != null)
+        {
+            // Create handler for content if it doesn't exist
+            if (content.Handler == null)
+            {
+                content.Handler = content.ToHandler(handler.MauiContext);
+            }
+
+            if (content.Handler?.PlatformView is SkiaView skiaContent)
+            {
+                handler.PlatformView.AddChild(skiaContent);
+            }
+        }
+    }
+}