feat(widgets): open a widget in its own window (#205)

Each widget in the Widgets panel (and the open widget's header) gets an
"Open in a new window" button, so a pad or board can sit beside the chat.
It opens the same filled-in URL the frame uses (widgetCompleteUrl, now
shared) with noopener,noreferrer, standalone as in Element. Only
cross-origin https URLs get the button: room state can carry anything,
e.g. a script URL from another client.

Refs #205

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/features/room/widgets/RoomWidgetView.tsx

@@ -11,7 +11,7 @@ import {
 import { Capability, ClientWidgetApi, IRoomEvent, Widget } from 'matrix-widget-api';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { GeneralWidgetDriver } from './GeneralWidgetDriver';
-import { WidgetPermissionRequest, isWidgetUrlSafe } from './widgetUtils';
+import { WidgetPermissionRequest, isWidgetUrlSafe, widgetCompleteUrl } from './widgetUtils';
 import { restrictWidgetMessages } from '../../../plugins/widgetTransport';
 import { WidgetPermissionPrompt } from './WidgetPermissionPrompt';
 import { isFromFilteredTimelineSet } from '../../../utils/room';
@@ -46,12 +46,7 @@ export function RoomWidgetView({ room, widget }: RoomWidgetViewProps) {
     if (!container) return undefined;
     const current = widgetRef.current;
 
-    const completeUrl = current.getCompleteUrl({
-      currentUserId: mx.getSafeUserId(),
-      widgetRoomId: room.roomId,
-      deviceId: mx.getDeviceId() ?? undefined,
-      baseUrl: mx.baseUrl,
-    });
+    const completeUrl = widgetCompleteUrl(current, mx, room.roomId);
 
     // Security: never render a same-origin widget with allow-same-origin (a
     // same-origin frame could break out of the sandbox against our own origin).