feat: chat and form for MAUI and Avalonia

[Shewart]

Summary

Two more ShellUI Blazor components that had no native version, each for MAUI and Avalonia. The registry now has 105 entries (102 CLI targets, 71 families).

• chat (Chat, ChatMessage, ChatInput): a conversation panel with header and footer slots and a message list that follows the newest message. Messages are styled by role (assistant, user, system), with a sender, timestamp, avatar and a typing indicator. The message box grows with its text and has a send button with a loading state.
• form (Form, FormField): fields with a label, description, required marker and error message. Validation stays with the app: it sets a field's Error, and the form reports whether it is valid and which field fails first.

Both demos show the new components, and the docs cover them.

Notes

• The message's name property is Sender on both platforms, because Avalonia controls already have a Name.
• On Avalonia, Enter sends and Shift+Enter starts a new line in the message box. On MAUI the button sends.
• The chat list keeps following a reply that grows while it arrives, unless the reader has scrolled up.
• A form field's error does not set the control's own HasError; the docs show how to set both.
• chat depends on button, which add chat installs.
• qr-code is not in this batch: it needs a QR-encoding package, and the CLI cannot install NuGet dependencies yet.

Verification

• Solution builds with 0 warnings and 0 errors on all four MAUI targets; 1002 tests pass; format is clean.
• Each component installs alone into a fresh Avalonia app, and both together into a fresh dotnet new maui app (Windows target), building with 0 warnings.
• MAUI demo checked on Windows in dark and light.
• Avalonia demo checked on Windows in dark and light with pointer and key input.