Serve the index page for an empty URL path instead of panicking (closes #149)

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

internal/server/export_test.go

@@ -0,0 +1,12 @@
+package server
+
+import "github.com/go-chi/chi/v5"
+
+// NewTestSPAServer creates a Server whose router serves only
+// the embedded web client.
+func NewTestSPAServer() *Server {
+	srv := &Server{router: chi.NewRouter()}
+	srv.setupSPA()
+
+	return srv
+}