SPAFileServer returns a http.Handler that calls h and falls back to serving the spa app container when h writes header http.StatusNotFound.

Root returns the content root subdirectory.

Output must be the relative path to where the frontend build too places the output index.html file.

Path needs to match <base href="/ui/"> in index.html and baseHref in angular.json.

go:embed all:holos/dist/holos/ui.