Add ChatGPT-style chat interface using MudBlazor components. User messages
display on the right (purple), bot echoes "success msg!" on the left.
Includes app bar, centered 768px layout, bottom-anchored messages, and
documented inline comments on every Blazor concept introduced.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
