feat: command, command palette and navigation menu for MAUI and Avalonia

[Shewart]

Summary

The second batch of ShellUI Blazor components that had no native version, each for MAUI and Avalonia. The registry now has 102 entries (99 CLI targets, 68 families).

• command (Command, CommandGroup, CommandItem, CommandSeparator): a search field over a list of grouped items. Typing filters by each item's text and keywords, and Enter picks the highlighted item.
• CommandPalette, in the same file: the same list in a dialog near the top of the page. It opens from code, and picking an item, the backdrop or Escape closes it.
• navigation-menu (NavigationMenu, NavigationMenuItem, NavigationMenuLink): a row of top-level links for an app header. An item with links opens a panel of them (title, description, icon); an item without is a plain link.

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

Notes

• On Avalonia, Up / Down move the command highlight, a hotkey (Ctrl+K by default) toggles the palette anywhere in the window, and the navigation menu works with the arrow keys like the menubar.
• On MAUI the class name Command matches Microsoft.Maui.Controls.Command. C# that imports both namespaces needs to qualify one; this is in the docs.

Verification

• Solution builds with 0 warnings and 0 errors on all four MAUI targets; 975 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.