shellui doctor: check a project's setup #55

Summary

Most setup problems are silent: a missing Tailwind CLI, a stale app.css, shellui.js not referenced, components installed but missing their dependencies, no interactive render mode. A doctor command would find them.

Proposal

shellui doctor checks and reports (with a fix for each):

• shellui.json present and valid; ComponentsPath exists
• Tailwind CLI available (shared cache or project copy) and app.css newer than input.css
• App.razor links app.css and shellui.js, and has the theme script
• an interactive render mode is set (components don't respond under static SSR)
• every installed component's dependencies are installed
• _Imports.razor has the needed @using lines
• package and CLI not both used in one project