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/Pages/DetailPage.xaml.cs

@@ -0,0 +1,34 @@
+using System;
+using Microsoft.Maui.Controls;
+
+namespace ShellDemo.Pages;
+
+[QueryProperty(nameof(ItemName), "item")]
+public partial class DetailPage : ContentPage
+{
+    private string _itemName = "Detail Item";
+
+    public string ItemName
+    {
+        get => _itemName;
+        set
+        {
+            _itemName = value;
+            Title = $"Detail: {value}";
+            if (ItemLabel != null)
+            {
+                ItemLabel.Text = $"You navigated to: {value}";
+            }
+        }
+    }
+
+    public DetailPage()
+    {
+        InitializeComponent();
+    }
+
+    private async void OnGoBackClicked(object? sender, EventArgs e)
+    {
+        await Shell.Current.GoToAsync("..");
+    }
+}