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/ButtonsPage.xaml.cs

@@ -0,0 +1,93 @@
+using System;
+using Microsoft.Maui.Controls;
+
+namespace ShellDemo.Pages;
+
+public partial class ButtonsPage : ContentPage
+{
+    private int _eventCount = 0;
+
+    public ButtonsPage()
+    {
+        InitializeComponent();
+    }
+
+    private void LogEvent(string message)
+    {
+        _eventCount++;
+        var timestamp = DateTime.Now.ToString("HH:mm:ss");
+        EventLog.Text = $"[{timestamp}] {_eventCount}. {message}\n{EventLog.Text}";
+    }
+
+    private void OnDefaultButtonClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Default Button clicked");
+    }
+
+    private void OnDefaultButtonPressed(object? sender, EventArgs e)
+    {
+        LogEvent("Default Button pressed");
+    }
+
+    private void OnDefaultButtonReleased(object? sender, EventArgs e)
+    {
+        LogEvent("Default Button released");
+    }
+
+    private void OnTextButtonClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Text Button clicked");
+    }
+
+    private void OnPrimaryClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Primary button clicked");
+    }
+
+    private void OnSuccessClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Success button clicked");
+    }
+
+    private void OnWarningClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Warning button clicked");
+    }
+
+    private void OnDangerClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Danger button clicked");
+    }
+
+    private void OnPurpleClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Purple button clicked");
+    }
+
+    private void OnEnabledClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Enabled button clicked");
+    }
+
+    private void OnToggleClicked(object? sender, EventArgs e)
+    {
+        DisabledButton.IsEnabled = !DisabledButton.IsEnabled;
+        DisabledButton.Text = DisabledButton.IsEnabled ? "Now Enabled!" : "Disabled Button";
+        LogEvent($"Toggled button to: {(DisabledButton.IsEnabled ? "Enabled" : "Disabled")}");
+    }
+
+    private void OnWideClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Wide button clicked");
+    }
+
+    private void OnTallClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Tall button clicked");
+    }
+
+    private void OnRoundClicked(object? sender, EventArgs e)
+    {
+        LogEvent("Round button clicked");
+    }
+}