Fix: Hide iframe until loaded to prevent flash

ts/routes/reviewer/Reviewer.svelte

@@ -32,5 +32,6 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
     iframe {
         width: 100%;
         height: 100%;
+        visibility: hidden;
     }
 </style>

ts/routes/reviewer/reviewer.ts

@@ -37,6 +37,7 @@ export class ReviewerState {
     iframe: HTMLIFrameElement | undefined = undefined;
 
     onReady() {
+        this.iframe!.style.visibility = "visible";
         this.showQuestion(null);
         addEventListener("message", this.onMessage.bind(this));
     }