fix: Jitsi meeting join rendered blank page — call JitsiMeetExternalAPI with 'new'

- Bind JitsiMeetExternalAPI as extension type external constructor;
  the old external function binding invoked the ES6 class without 'new'
  and threw at runtime (silently, in release builds)
- joinMeeting: wait up to 5s for platform-view container div (post-frame
  race), build options via setProperty (jsify cannot carry DOM nodes),
  return success bool, log failures to browser console
- jitsi_screen: show 'Could not start the meeting' UI instead of blank
- v1.6.2+10, deployed to app2.m8chat.au

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pubspec.yaml

@@ -1,7 +1,7 @@
 name: m8chat_app
 description: "M8Chat — Matrix chat client for Android, iOS, and Web."
 publish_to: 'none'
-version: 1.6.0+8
+version: 1.6.2+10
 
 environment:
   sdk: '>=3.11.0 <4.0.0'