setupSPA took the file name from request.URL.Path[1:], which panics
when the path is empty, as it is for an absolute-form request line
with no path ("GET http://host HTTP/1.1"). It now trims a leading "/"
instead; an empty name fails to read, so the request gets the index
page, as "/" already does.
The new test in internal/server sends such a request and expects 200
and the index page. export_test.go gives it a Server whose router has
only the web client route.
Model: opus-5-5
