fix(calls): read the EC iframe document defensively when the widget failed to load

When the Element Call frame's navigation fails (offline, blocked) it becomes a
cross-origin error page and `iframe.contentWindow.document` throws a
SecurityError. Every DOM-driven call hook (CallControl selectors, deafen DOM
fallback, useCallSpeakers/useRemoteAllMuted observers, CallEmbed theme
injection) read it unguarded, so the moment the 25 s load watchdog fired the
page logged a burst of uncaught SecurityErrors. Reproduced headless by aborting
the EC index request. One `getCallDocument()` helper now treats "can't read" as
"not loaded yet"; the watchdog overlay itself was already correct.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/plugins/call/CallControl.ts

@@ -2,6 +2,7 @@ import { ClientWidgetApi } from 'matrix-widget-api';
 import { EventEmitter } from 'events';
 import { CallControlState } from './CallControlState';
 import { ElementMediaStateDetail, ElementMediaStatePayload, ElementWidgetActions } from './types';
+import { getCallDocument } from './utils';
 
 export enum CallControlEvent {
   StateUpdate = 'state_update',
@@ -62,7 +63,7 @@ export class CallControl extends EventEmitter implements CallControlState {
   private joined = false;
 
   private get document(): Document | undefined {
-    return this.iframe.contentDocument ?? this.iframe.contentWindow?.document;
+    return getCallDocument(this.iframe);
   }
 
   private get screenshareButton(): HTMLElement | undefined {
@@ -265,7 +266,7 @@ export class CallControl extends EventEmitter implements CallControlState {
   }
 
   private setSound(sound: boolean): void {
-    const callDocument = this.iframe.contentDocument ?? this.iframe.contentWindow?.document;
+    const callDocument = getCallDocument(this.iframe);
     if (callDocument) {
       callDocument.querySelectorAll('audio').forEach((el) => {
         const isScreenshareAudio = el.getAttribute('data-lk-source') === 'screen_share_audio';
@@ -277,7 +278,7 @@ export class CallControl extends EventEmitter implements CallControlState {
 
   private applyScreenshareAudioMuted(): void {
     if (!this.sound) return;
-    const callDocument = this.iframe.contentDocument ?? this.iframe.contentWindow?.document;
+    const callDocument = getCallDocument(this.iframe);
     if (callDocument) {
       callDocument
         .querySelectorAll<HTMLAudioElement>('audio[data-lk-source="screen_share_audio"]')