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

@@ -0,0 +1,47 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="ShellDemo.Pages.DetailPage"
+             Title="Detail Page"
+             BackgroundColor="{StaticResource PageBackground}">
+
+    <VerticalStackLayout Padding="30"
+                         Spacing="20"
+                         VerticalOptions="Center">
+
+        <Label Text="Pushed Page"
+               FontSize="28"
+               FontAttributes="Bold"
+               HorizontalOptions="Center"
+               TextColor="{StaticResource PurpleColor}" />
+
+        <Label x:Name="ItemLabel"
+               Text="You navigated to: Detail Item"
+               FontSize="16"
+               HorizontalOptions="Center" />
+
+        <Label Text="This page was pushed onto the navigation stack using Shell.Current.GoToAsync()"
+               FontSize="14"
+               TextColor="{StaticResource TextSecondary}"
+               HorizontalTextAlignment="Center"
+               LineBreakMode="WordWrap" />
+
+        <BoxView HeightRequest="2"
+                 Color="#E0E0E0"
+                 Margin="0,20" />
+
+        <Button Text="Go Back (Pop)"
+                BackgroundColor="{StaticResource PurpleColor}"
+                TextColor="White"
+                HorizontalOptions="Center"
+                Padding="30,10"
+                Clicked="OnGoBackClicked" />
+
+        <Label Text="Use the back button above or the hardware/gesture back to pop this page"
+               FontSize="12"
+               TextColor="{StaticResource TextSecondary}"
+               HorizontalTextAlignment="Center"
+               Margin="0,20,0,0" />
+
+    </VerticalStackLayout>
+</ContentPage>