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/presentation/jitsi_screen.dart

@@ -0,0 +1,121 @@
+// Version: 1.2.0 | Created: 2026-04-05 | Updated: 2026-04-10
+// Full-screen Jitsi meeting embedded via HtmlElementView (web platform view).
+
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+import '../data/jitsi_web_service.dart';
+import '../domain/jitsi_link.dart';
+
+class JitsiScreen extends StatefulWidget {
+  const JitsiScreen({super.key, required this.meetingUrl});
+
+  /// The raw meeting link or room name provided by the user.
+  final String meetingUrl;
+
+  @override
+  State<JitsiScreen> createState() => _JitsiScreenState();
+}
+
+class _JitsiScreenState extends State<JitsiScreen> {
+  late final JitsiLink? _link;
+  bool _meetingStarted = false;
+  bool _meetingEnded = false;
+
+  @override
+  void initState() {
+    super.initState();
+    _link = JitsiLink.tryParse(widget.meetingUrl);
+    if (_link != null) {
+      JitsiWebService.instance.ensureViewRegistered();
+    }
+  }
+
+  @override
+  void dispose() {
+    JitsiWebService.instance.dispose();
+    super.dispose();
+  }
+
+  void _startMeeting() {
+    if (_link == null || _meetingStarted) return;
+    _meetingStarted = true;
+
+    // Post-frame so the HtmlElementView div is mounted in the DOM.
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      JitsiWebService.instance.joinMeeting(
+        roomName: _link.roomName,
+        jwt: _link.jwt,
+        onReadyToClose: () {
+          if (mounted) {
+            setState(() => _meetingEnded = true);
+          }
+        },
+      );
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (_link == null) {
+      return Scaffold(
+        appBar: AppBar(title: const Text('Conference')),
+        body: Center(
+          child: Padding(
+            padding: const EdgeInsets.all(32),
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: [
+                const Icon(Icons.link_off, size: 48),
+                const SizedBox(height: 16),
+                const Text('Could not parse the meeting link.'),
+                const SizedBox(height: 24),
+                ElevatedButton(
+                  onPressed: () => context.go('/rooms'),
+                  child: const Text('Back'),
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+    }
+
+    if (_meetingEnded) {
+      return Scaffold(
+        body: Center(
+          child: Padding(
+            padding: const EdgeInsets.all(32),
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: [
+                Icon(
+                  Icons.videocam_off_outlined,
+                  size: 64,
+                  color: Theme.of(context).colorScheme.primary,
+                ),
+                const SizedBox(height: 16),
+                Text(
+                  'Meeting ended',
+                  style: Theme.of(context).textTheme.headlineSmall,
+                ),
+                const SizedBox(height: 24),
+                ElevatedButton(
+                  onPressed: () => context.go('/rooms'),
+                  child: const Text('Back to M8Chat'),
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+    }
+
+    // Render the Jitsi iframe and kick off the meeting once mounted.
+    _startMeeting();
+
+    return Scaffold(
+      body: const HtmlElementView(viewType: JitsiWebService.viewType),
+    );
+  }
+}