Add TodoApp sample with reconstructed XAML

Complete TodoApp sample application with:
- App.xaml/cs: Colors and styles for light/dark themes
- TodoListPage: Task list with theme toggle switch
- NewTodoPage: Form to create new tasks
- TodoDetailPage: Edit task details with delete option
- TodoItem.cs/TodoService.cs: Data model and service
- SVG icons for save, delete, and add actions

Theme switching via toggle on main page applies app-wide.

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

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

samples/TodoApp/Resources/Images/icon_add_light.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#5C6BC0" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <line x1="12" y1="5" x2="12" y2="19"></line>
+  <line x1="5" y1="12" x2="19" y2="12"></line>
+</svg>