Scaffolded host strips @source "./_scan/", so packaged modules' Tailwind classes are never compiled #290

Summary

HostTemplates.AppCss() removes the @source "./_scan/" line when scaffolding:

// cli/SimpleModule.Cli/Templates/HostTemplates.cs
var lines = EmbeddedResourceReader.ReadTemplateLines("Templates.Host.Styles.app.css");
lines.RemoveAll(line => line.Contains("_scan/", StringComparison.Ordinal));

Styles/_scan/ is where the CollectModuleAssets MSBuild target stages every module's
built .pages.js. It is the only way a packaged (NuGet) module's utility classes
reach the compiled CSS — a packaged module has no .tsx under src/modules/, so the
@source "../../modules/**/Pages/**/*.tsx" glob cannot see it.

With the directive stripped, a scaffolded host stages those bundles on every build and
then scans none of them.

Reproduction

sm new project MyApp && cd MyApp
sm install SimpleModule.Users
npm run build && dotnet build src/MyApp.Host

Any utility class used only by the packaged module's pages is absent from
src/MyApp.Host/wwwroot/css/app.css.

Why it matters

Same silent-failure shape as #288: the page renders, some classes work (whatever another
package happens to use literally), and the rest do nothing. It reads as a broken module
rather than a missing @source root.

Note on the fix

The strip looks deliberate — a fresh scaffold has no Styles/_scan/ directory yet, and
Tailwind errors on an @source path that does not exist. So restoring the directive
likely needs the scaffold to create Styles/_scan/ (e.g. with a .gitkeep) so the first
npm run build succeeds before any dotnet build has staged anything there.

Found while fixing #288 in #289; the Tailwind up-to-date check already tracks these
staged bundles, so only the missing @source root remains.