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>

samples/TodoApp/App.cs

@@ -0,0 +1,21 @@
+// TodoApp - Main Application with NavigationPage
+
+using Microsoft.Maui.Controls;
+
+namespace TodoApp;
+
+public class App : Application
+{
+    public static NavigationPage? NavigationPage { get; private set; }
+
+    public App()
+    {
+        NavigationPage = new NavigationPage(new TodoListPage())
+        {
+            Title = "OpenMaui Todo App",
+            BarBackgroundColor = Color.FromArgb("#2196F3"),
+            BarTextColor = Colors.White
+        };
+        MainPage = NavigationPage;
+    }
+}