fix(security): widget API only accepts messages from the widget's own frame

matrix-widget-api's host transport handled a message from ANY window on the
page as long as it carried the widget's id; its strictOriginCheck only
compares with the host's own origin and is off by default. The call's id is
the fixed 'call-embed', so any other frame (a room widget, a URL-preview
embed) could post fromWidget actions as the call. Reproduced locally: an
opaque-origin frame posting one io.lotus.hotkey keydown for the PTT key
turned a push-to-talk user's mic on ("● Live").

restrictWidgetMessages() swaps each ClientWidgetApi transport's listener
for one that requires ev.source === the widget iframe's window and
ev.origin === the widget's origin. Applied to the call and to room widgets
(so one widget can't impersonate another). Verified: the spoof no longer
opens the mic; PTT/deafen from inside the call, screenshare, speaking
indicator and room widgets (capability prompt, send, live events) unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/plugins/call/CallEmbed.ts

@@ -29,6 +29,7 @@ import { CallControl } from './CallControl';
 import { CallControlState } from './CallControlState';
 import { verifyDenoiseAssets } from './denoiseSmokeCheck';
 import { canDelegateCapability } from './utils';
+import { restrictWidgetMessages } from '../widgetTransport';
 
 // Maximum time to wait for the embedded Element Call iframe to progress from
 // initial load to a ready/joined state. If it hasn't by then, we assume the
@@ -305,6 +306,8 @@ export class CallEmbed {
 
     const callWidgetDriver: WidgetDriver = new CallWidgetDriver(mx, room.roomId);
     const call: ClientWidgetApi = new ClientWidgetApi(widget, iframe, callWidgetDriver);
+    // Only messages from the call's own frame and origin reach the widget API.
+    restrictWidgetMessages(call, iframe, widget.origin);
 
     this.mx = mx;
     this.call = call;