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/ProgressPage.xaml.cs

@@ -0,0 +1,84 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.Maui.Controls;
+
+namespace ShellDemo.Pages;
+
+public partial class ProgressPage : ContentPage
+{
+    private int _eventCount = 0;
+    private bool _isAnimating = false;
+
+    public ProgressPage()
+    {
+        InitializeComponent();
+    }
+
+    private void LogEvent(string message)
+    {
+        _eventCount++;
+        var timestamp = DateTime.Now.ToString("HH:mm:ss");
+        EventLog.Text = $"[{timestamp}] {_eventCount}. {message}\n{EventLog.Text}";
+    }
+
+    private void OnToggleIndicatorClicked(object? sender, EventArgs e)
+    {
+        ToggleIndicator.IsRunning = !ToggleIndicator.IsRunning;
+        LogEvent($"ActivityIndicator: {(ToggleIndicator.IsRunning ? "Started" : "Stopped")}");
+    }
+
+    private void OnProgressSliderChanged(object? sender, ValueChangedEventArgs e)
+    {
+        var value = e.NewValue / 100.0;
+        AnimatedProgress.Progress = value;
+        ProgressLabel.Text = $"Progress: {e.NewValue:0}%";
+    }
+
+    private void OnResetClicked(object? sender, EventArgs e)
+    {
+        AnimatedProgress.Progress = 0;
+        ProgressSlider.Value = 0;
+        LogEvent("Progress reset to 0%");
+    }
+
+    private async void OnAnimateClicked(object? sender, EventArgs e)
+    {
+        if (_isAnimating) return;
+        _isAnimating = true;
+        LogEvent("Animation started");
+
+        for (int i = (int)ProgressSlider.Value; i <= 100; i += 5)
+        {
+            AnimatedProgress.Progress = i / 100.0;
+            ProgressSlider.Value = i;
+            await Task.Delay(100);
+        }
+
+        _isAnimating = false;
+        LogEvent("Animation completed");
+    }
+
+    private async void OnSimulateClicked(object? sender, EventArgs e)
+    {
+        if (_isAnimating) return;
+        _isAnimating = true;
+        LogEvent("Download simulation started");
+
+        AnimatedProgress.Progress = 0;
+        ProgressSlider.Value = 0;
+
+        var random = new Random();
+        double progress = 0;
+        while (progress < 1.0)
+        {
+            progress += random.NextDouble() * 0.1;
+            if (progress > 1.0) progress = 1.0;
+            AnimatedProgress.Progress = progress;
+            ProgressSlider.Value = progress * 100;
+            await Task.Delay(200 + random.Next(300));
+        }
+
+        _isAnimating = false;
+        LogEvent("Download simulation completed");
+    }
+}