Docs: using ShellUI from a Razor class library shared by several hosts #53

Summary

Putting ShellUI components in a Razor class library shared by a Blazor Server app and a MAUI Blazor Hybrid app works, but the setup isn't documented and several steps aren't obvious.

What a guide should cover

• Hosts reference _content/<Library>/app.css and _content/<Library>/shellui.js, and must not copy the CSS into their own wwwroot.
• Tailwind scans only the library, so classes used in a host's own markup need an @source line in input.css.
• ThemeToggle assumes dark when nothing is stored and sets the dark class only after the first render, so each host needs the inline theme script that shellui init writes into a normal app's App.razor.
• Hosts that keep the session in browser storage need prerender: false.

Possible CLI help

shellui init could detect a Razor class library (Microsoft.NET.Sdk.Razor with no Program.cs) and print these host steps.