feat: Phase 3 — E2EE calls, Jitsi conferencing, help tab, web security model

- LiveKit call E2EE: CallE2EEManager exchanges encryption keys via Matrix
  to-device events (m.rtc.encryption_keys) for interop with Element X
- Olm bootstrapped in index.html before Flutter init; main.dart logs result
- Encrypted messages shown with lock icon and informative fallback text
- Profile screen: key restore dialog + security setup (cross-signing/backup)
- Jitsi feature: welcome screen (public, no login), conference tab, full-screen
  embed via JitsiMeetExternalAPI, JitsiLink parser for all common link formats
- Help tab: expandable cards for encryption, video calls, account management
- Web security model: no session persistence — device ID only across visits
- Media auth: MSC3916 authenticated endpoint for avatars (Synapse 1.120+)
- Router: welcome route as public landing page; jitsi route as public
- Manifest/index.html: M8Chat branding, dark theme colours

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/features/jitsi/data/jitsi_web_service.dart

@@ -0,0 +1,168 @@
+// Version: 1.1.0 | Created: 2026-04-05 | Updated: 2026-04-10
+// Web implementation: uses JitsiMeetExternalAPI via dart:js_interop.
+// This file is web-only — guarded by the Flutter web build.
+// The external_api.js script is lazy-loaded on first use — not in index.html.
+// This prevents privacy browsers (Brave, Mullvad) from blocking the app
+// at startup due to a cross-origin script from conf.m8chat.au.
+
+import 'dart:async';
+import 'dart:js_interop';
+import 'dart:ui_web' as ui_web;
+
+import 'package:web/web.dart' as web;
+
+import '../../../core/config/app_config.dart';
+
+/// Manages a single Jitsi iframe session via the JitsiMeetExternalAPI.
+class JitsiWebService {
+  JitsiWebService._();
+  static final instance = JitsiWebService._();
+
+  JSObject? _api;
+  bool _viewRegistered = false;
+  bool _scriptLoaded = false;
+  bool _scriptLoading = false;
+
+  /// Unique view type for HtmlElementView.
+  static const String viewType = 'jitsi-meet-container';
+
+  /// Lazy-loads the JitsiMeetExternalAPI script from conf.m8chat.au.
+  /// Returns true if the script loaded successfully, false otherwise.
+  Future<bool> _ensureScriptLoaded() async {
+    if (_scriptLoaded) return true;
+    if (_scriptLoading) {
+      // Wait for in-flight load to finish.
+      for (var i = 0; i < 50; i++) {
+        await Future<void>.delayed(const Duration(milliseconds: 100));
+        if (_scriptLoaded) return true;
+      }
+      return false;
+    }
+    _scriptLoading = true;
+
+    final completer = Completer<bool>();
+    final script =
+        web.document.createElement('script') as web.HTMLScriptElement;
+    script.src = 'https://${AppConfig.jitsiDomain}/external_api.js';
+    script.async = true;
+    script.onload = (web.Event event) {
+      _scriptLoaded = true;
+      _scriptLoading = false;
+      completer.complete(true);
+    }.toJS;
+    script.onerror = (web.Event event) {
+      _scriptLoading = false;
+      completer.complete(false);
+    }.toJS;
+    web.document.head?.append(script);
+
+    return completer.future;
+  }
+
+  /// Registers the platform view factory (once).
+  void ensureViewRegistered() {
+    if (_viewRegistered) return;
+    ui_web.platformViewRegistry.registerViewFactory(
+      viewType,
+      (int viewId, {Object? params}) {
+        final div = web.document.createElement('div') as web.HTMLDivElement;
+        div.id = 'jitsi-container-$viewId';
+        div.style
+          ..width = '100%'
+          ..height = '100%';
+        return div;
+      },
+    );
+    _viewRegistered = true;
+  }
+
+  /// Starts a Jitsi meeting inside the container div created by the platform view.
+  /// Call this AFTER the HtmlElementView has been mounted (e.g. in a post-frame callback).
+  /// Lazy-loads the Jitsi script on first call.
+  Future<void> joinMeeting({
+    required String roomName,
+    String? jwt,
+    String? displayName,
+    String? avatarUrl,
+    void Function()? onReadyToClose,
+  }) async {
+    dispose(); // tear down any previous meeting
+
+    // Lazy-load the external_api.js script if not already present.
+    final loaded = await _ensureScriptLoaded();
+    if (!loaded) return;
+
+    // Find the container div — there should be exactly one with our prefix.
+    final containers = web.document.querySelectorAll('[id^="jitsi-container-"]');
+    if (containers.length == 0) return;
+    final parentNode = containers.item(containers.length - 1);
+    if (parentNode == null) return;
+
+    final configOverwrite = <String, Object?>{
+      'startAudioMuted': 0,
+      'startVideoMuted': 0,
+      'disableDeepLinking': true,
+      'prejoinPageEnabled': true,
+    }.jsify();
+
+    final interfaceConfigOverwrite = <String, Object?>{
+      'SHOW_CHROME_EXTENSION_BANNER': false,
+    }.jsify();
+
+    final options = <String, Object?>{
+      'roomName': roomName,
+      'parentNode': parentNode,
+      'width': '100%',
+      'height': '100%',
+      'configOverwrite': configOverwrite,
+      'interfaceConfigOverwrite': interfaceConfigOverwrite,
+    };
+
+    if (jwt != null && jwt.isNotEmpty) {
+      options['jwt'] = jwt;
+    }
+
+    if (displayName != null && displayName.isNotEmpty) {
+      options['userInfo'] = <String, Object?>{
+        'displayName': displayName,
+        if (avatarUrl != null) 'avatarUrl': avatarUrl,
+      }.jsify();
+    }
+
+    final jsOptions = options.jsify();
+    _api = _createJitsiApi(AppConfig.jitsiDomain.toJS, jsOptions as JSObject);
+
+    if (onReadyToClose != null && _api != null) {
+      _addEventListener(_api!, 'readyToClose'.toJS, () {
+        onReadyToClose();
+      }.toJS);
+    }
+  }
+
+  /// Cleans up the Jitsi iframe.
+  void dispose() {
+    if (_api != null) {
+      _disposeApi(_api!);
+      _api = null;
+    }
+  }
+}
+
+/// Calls `new JitsiMeetExternalAPI(domain, options)`.
+@JS('JitsiMeetExternalAPI')
+external JSObject _createJitsiApi(JSString domain, JSObject options);
+
+/// Calls `api.addEventListener(event, callback)`.
+@JS()
+extension type _JitsiApi(JSObject _) implements JSObject {
+  external void addEventListener(JSString event, JSFunction callback);
+  external void dispose();
+}
+
+void _addEventListener(JSObject api, JSString event, JSFunction callback) {
+  (api as _JitsiApi).addEventListener(event, callback);
+}
+
+void _disposeApi(JSObject api) {
+  (api as _JitsiApi).dispose();
+}