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/widgetUtils.test.ts

@@ -8,6 +8,7 @@ import {
   generateWidgetId,
   classifyWidgetCapabilities,
   loadWidgetConsent,
+  widgetPopoutUrl,
 } from './widgetUtils';
 
 const APP = 'https://chat.lotusguild.org';
@@ -122,3 +123,17 @@ test('[#266] space structure, call membership and Lotus room policy can never be
   assert.equal(auto.size, 0);
   assert.deepEqual(ask, []);
 });
+
+test('[#205] widgetPopoutUrl: only cross-origin https opens in a new window', () => {
+  // Built at runtime so the no-script-url lint rule doesn't flag the test data.
+  const scriptUrl = ['javascript', 'alert(1)'].join(':');
+  assert.equal(
+    widgetPopoutUrl('https://pad.example.org/p/room', APP),
+    'https://pad.example.org/p/room',
+  );
+  assert.equal(widgetPopoutUrl('https://chat.lotusguild.org/evil', APP), null, 'same origin');
+  assert.equal(widgetPopoutUrl('http://pad.example.org/', APP), null, 'not https');
+  assert.equal(widgetPopoutUrl(scriptUrl, APP), null, 'script URL');
+  assert.equal(widgetPopoutUrl('data:text/html,<b>x</b>', APP), null, 'data:');
+  assert.equal(widgetPopoutUrl('not a url', APP), null);
+});