Add WebView support via WebKitGTK

Implements Priority 4 item: WebView via WebKitGTK

New files:
- Interop/WebKitGtk.cs - P/Invoke bindings for WebKitGTK library
- Views/LinuxWebView.cs - WebKitGTK-based WebView platform control
- Handlers/WebViewHandler.Linux.cs - MAUI handler for WebView on Linux
- samples_temp/WebViewDemo/ - Demo app for WebView functionality

Features:
- Full HTML5/CSS3/JavaScript support via WebKitGTK
- Navigation (back/forward/reload)
- URL and HTML source loading
- JavaScript evaluation
- Navigation events (Navigating/Navigated)
- Automatic GTK event processing

Requirements:
- libwebkit2gtk-4.1-0 package on target Linux system

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

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

samples_temp/ShellDemo/Pages/DetailPage.cs

@@ -0,0 +1,123 @@
+// DetailPage - Demonstrates push/pop navigation
+
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Graphics;
+using Microsoft.Maui.Platform.Linux.Hosting;
+
+namespace ShellDemo;
+
+/// <summary>
+/// A detail page that can be pushed onto the navigation stack.
+/// </summary>
+public class DetailPage : ContentPage
+{
+    private readonly string _itemName;
+
+    public DetailPage() : this("Detail Item")
+    {
+    }
+
+    public DetailPage(string itemName)
+    {
+        _itemName = itemName;
+        Title = "Detail Page";
+
+        Content = new VerticalStackLayout
+        {
+            Padding = new Thickness(30),
+            Spacing = 20,
+            VerticalOptions = LayoutOptions.Center,
+            Children =
+            {
+                new Label
+                {
+                    Text = "Pushed Page",
+                    FontSize = 28,
+                    FontAttributes = FontAttributes.Bold,
+                    HorizontalOptions = LayoutOptions.Center,
+                    TextColor = Color.FromArgb("#9C27B0")
+                },
+
+                new Label
+                {
+                    Text = $"You navigated to: {_itemName}",
+                    FontSize = 16,
+                    HorizontalOptions = LayoutOptions.Center
+                },
+
+                new Label
+                {
+                    Text = "This page was pushed onto the navigation stack using Shell.Current.GoToAsync()",
+                    FontSize = 14,
+                    TextColor = Colors.Gray,
+                    HorizontalTextAlignment = TextAlignment.Center,
+                    LineBreakMode = LineBreakMode.WordWrap
+                },
+
+                new BoxView
+                {
+                    HeightRequest = 2,
+                    Color = Color.FromArgb("#E0E0E0"),
+                    Margin = new Thickness(0, 20)
+                },
+
+                CreateBackButton(),
+
+                new Label
+                {
+                    Text = "Use the back button above or the hardware/gesture back to pop this page",
+                    FontSize = 12,
+                    TextColor = Colors.Gray,
+                    HorizontalTextAlignment = TextAlignment.Center,
+                    Margin = new Thickness(0, 20, 0, 0)
+                }
+            }
+        };
+    }
+
+    private Button CreateBackButton()
+    {
+        var backBtn = new Button
+        {
+            Text = "Go Back (Pop)",
+            BackgroundColor = Color.FromArgb("#9C27B0"),
+            TextColor = Colors.White,
+            HorizontalOptions = LayoutOptions.Center,
+            Padding = new Thickness(30, 10)
+        };
+
+        backBtn.Clicked += (s, e) =>
+        {
+            // Pop this page off the navigation stack using LinuxViewRenderer
+            Console.WriteLine("[DetailPage] Go Back clicked");
+            var success = LinuxViewRenderer.PopPage();
+            Console.WriteLine($"[DetailPage] PopPage result: {success}");
+        };
+
+        return backBtn;
+    }
+}
+
+/// <summary>
+/// Query property for passing data to DetailPage.
+/// </summary>
+[QueryProperty(nameof(ItemName), "item")]
+public class DetailPageWithQuery : DetailPage
+{
+    private string _itemName = "Item";
+
+    public string ItemName
+    {
+        get => _itemName;
+        set
+        {
+            _itemName = value;
+            // Update the title when the property is set
+            Title = $"Detail: {value}";
+        }
+    }
+
+    public DetailPageWithQuery() : base()
+    {
+    }
+}