fix: front-end improvements

makes some fields visible and other hidden depending on the result of
some API calls, e.g. visitor is logged in -> show "Log out" button

WebApp/wwwroot/css/panel.css

@@ -4,6 +4,11 @@
     font-size: 36px;
 }
 
+.hidden-before-load {
+    display: none !important;
+    visibility: hidden !important;
+}
+
 body {
     font-family: 'Segoe UI', sans-serif;
 }