feat: button group, input group and menubar for MAUI and Avalonia

[Shewart]

Summary

The first batch of ShellUI Blazor components that had no native version, each for MAUI and Avalonia. The registry now has 100 entries (97 CLI targets, 66 families).

• button-group (ButtonGroup): joins its buttons into one control. Only the outer corners are rounded and neighbouring borders overlap, so Outline buttons share one line. Horizontal or vertical.
• input-group (InputGroup): an input with an icon, a short text or any view before or after the text, inside the same border. Same height, focus ring and error state as Input.
• menubar (Menubar, MenubarMenu, MenubarItem, MenubarSeparator): a row of menus, each opening a panel of items under its title, with optional icons and shortcut hints.

Both demos show the new components, and the docs cover them, including how the Avalonia versions differ.

Notes

• Button gains an internal corner override that ButtonGroup sets. A project with an older button needs shellui-native update button before adding button-group; this is in the docs and belongs in the next release notes.
• On Avalonia the menubar titles are tab stops: Left / Right move between them, Enter / Space / Down open a menu, and Up / Down move inside it.
• InputGroup has Keyboard on MAUI only.

Verification

• Solution builds with 0 warnings and 0 errors on all four MAUI targets; 957 tests pass; format is clean.
• Each component installs alone into a fresh Avalonia app, and all three 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.