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/domain/jitsi_link.dart

@@ -0,0 +1,43 @@
+// Version: 1.0.0 | Created: 2026-04-05
+// Parses a Jitsi meeting link into room name and optional JWT.
+
+/// Parsed Jitsi meeting link.
+class JitsiLink {
+  const JitsiLink({required this.roomName, this.jwt});
+
+  final String roomName;
+  final String? jwt;
+
+  /// Parses a Jitsi link in common formats:
+  /// - `https://conf.m8chat.au/RoomName`
+  /// - `https://conf.m8chat.au/RoomName?jwt=abc123`
+  /// - `conf.m8chat.au/RoomName?jwt=abc123`
+  /// - `RoomName` (bare room name)
+  static JitsiLink? tryParse(String input) {
+    final trimmed = input.trim();
+    if (trimmed.isEmpty) return null;
+
+    // Try as a URL first.
+    var uriString = trimmed;
+    if (!uriString.contains('://') && uriString.contains('/')) {
+      uriString = 'https://$uriString';
+    }
+
+    final uri = Uri.tryParse(uriString);
+    if (uri != null && uri.pathSegments.isNotEmpty && uri.host.isNotEmpty) {
+      final roomName = uri.pathSegments
+          .where((s) => s.isNotEmpty)
+          .join('/');
+      if (roomName.isEmpty) return null;
+      final jwt = uri.queryParameters['jwt'];
+      return JitsiLink(roomName: roomName, jwt: jwt);
+    }
+
+    // Bare room name — no slashes, no protocol.
+    if (!trimmed.contains('/') && !trimmed.contains(' ')) {
+      return JitsiLink(roomName: trimmed);
+    }
+
+    return null;
+  }
+}