Fix session detection for "/" endpoint

Users who have not signed in are now properly redirected to the login
page.

serve.js

@@ -194,7 +194,7 @@ app.post('/pdf', async (req, res) => {
 });
 
 app.get('/', async (req, res) => {
-    if(typeof(req.session) != "undefined") {
+    if (req.session.username) {
         res.redirect('/home.html');
     } else {
         res.redirect('/login.html');