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/routes.go

@@ -3,6 +3,7 @@ package server
 import (
 	"io/fs"
 	"net/http"
+	"strings"
 	"time"
 
 	"sneak.berlin/go/neoirc/web"
@@ -137,8 +138,10 @@ func (srv *Server) setupSPA() {
 			return
 		}
 
+		// The path is empty, not "/", for a request line such
+		// as "GET http://host HTTP/1.1".
 		fileData, readErr := readFS.ReadFile(
-			request.URL.Path[1:],
+			strings.TrimPrefix(request.URL.Path, "/"),
 		)
 		if readErr != nil || len(fileData) == 0 {
 			indexHTML, indexErr := readFS.ReadFile(