Add ShellDemo sample with comprehensive XAML controls showcase

Complete ShellDemo application demonstrating all MAUI controls:
- App/AppShell: Shell navigation with flyout menu
- HomePage: Feature cards, theme toggle, quick actions
- ButtonsPage: Button styles, states, variations, event logging
- TextInputPage: Entry, Editor, SearchBar with keyboard shortcuts
- SelectionPage: CheckBox, Switch, Slider with colored variants
- PickersPage: Picker, DatePicker, TimePicker demos
- ListsPage: CollectionView with fruits, colors, contacts
- ProgressPage: ProgressBar, ActivityIndicator, interactive demo
- GridsPage: Grid layouts - auto/star/absolute sizing, spans, nesting
- AboutPage: OpenMaui Linux information
- DetailPage: Push/pop navigation demo

All pages use proper XAML with code-behind following MAUI patterns.

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

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

samples/ShellDemo/MauiProgram.cs

@@ -0,0 +1,24 @@
+using Microsoft.Maui;
+using Microsoft.Maui.Controls.Hosting;
+using Microsoft.Maui.Hosting;
+using Microsoft.Maui.Platform.Linux.Hosting;
+
+namespace ShellDemo;
+
+public static class MauiProgram
+{
+    public static MauiApp CreateMauiApp()
+    {
+        var builder = MauiApp.CreateBuilder();
+        builder
+            .UseMauiApp<App>()
+            .UseLinuxPlatform()
+            .ConfigureFonts(fonts =>
+            {
+                fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
+                fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
+            });
+
+        return builder.Build();
+    }
+}