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>

help4bis-claude-notes/decisions.md

@@ -75,3 +75,10 @@
 - **Audit results:** Backend all healthy (client API, livekit JWT, Jitsi external_api.js, push gateway, TURN 3478/5349). Full E2E pass in headless Chromium: login, rooms, spaces, profile, conference, help tabs, room open, message send verified on Synapse. Disposable audit users created and erased via admin API.
 - **Files affected:** assets/images/m8logo.svg, lib/app/theme.dart (2.0.0), lib/core/config/app_config.dart (2.1.0), web/manifest.json, pubspec.yaml (1.6.0+8), em-dash fixes in 8 presentation/util files, chat_screen.dart lint fix (_leaveRoom context param).
 - **Status:** Done — built, deployed to app2.m8chat.au (backup public_html.bak.20260703_pre160.tar.gz), visually verified light+dark.
+
+### 2026-07-04 07:15 — Jitsi conference join fixed (v1.6.2+10)
+- **Decision:** Root cause of "blank page on join" was the JS interop binding calling JitsiMeetExternalAPI (an ES6 class) as a plain function, i.e. without `new`. Browsers throw "Class constructor cannot be invoked without 'new'" and the meeting screen stayed blank. Feature had never worked in the web release build.
+- **Fix:** `_JitsiApi` extension type with an external constructor (compiles to `new`), container-div wait loop in joinMeeting (post-frame callback could fire before the platform view div existed), JS options built via setProperty instead of Map.jsify (DOM node in a jsify map is unsupported), joinMeeting returns bool, jitsi_screen shows an error screen instead of blank on failure.
+- **Verified:** headless Chromium on production build: iframe renders, prejoin page shows, devices pass, name entry works, reaches the Prosody JWT auth gate (expected for a tokenless room). COEP require-corp turned out NOT to block conf.m8chat.au in current Chromium.
+- **Files affected:** lib/features/jitsi/data/jitsi_web_service.dart (1.2.0), lib/features/jitsi/presentation/jitsi_screen.dart (1.3.0), app_config 1.6.2, pubspec 1.6.2+10.
+- **Status:** Done — deployed to app2.m8chat.au (backups pre160/pre161 tarballs on server).