Add Icon Support to TreeData

[emncnozge]

💡 What is the current behavior?

TreeData does not support an Icon property, so tree nodes cannot render icons.

🆕 What is the new behavior?

• Added Icon property to TreeData
• Updated README with example usage.
• Added serialization test for TreeData including the Icon field.
• Implemented icon rendering in JS interop.

🏁 Checklist

A pull request can only be merged if all of these conditions are met (where applicable):

• 🦮 Accessibility (a11y) features were implemented
• 🗺️ Internationalization (i18n) - no hard coded strings
• 📲 Responsiveness - components handle viewport changes and content overflow gracefully
• 📄 Documentation was reviewed/updated
• 🧪 Unit tests were added/updated and pass (dotnet test)
• 🏗️ Successful compilation (dotnet build, changes pushed)