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/rooms/presentation/rooms_screen.dart

@@ -1,10 +1,12 @@
-// Version: 1.1.0 | Created: 2026-04-01
+// Version: 1.3.0 | Created: 2026-04-01 | Updated: 2026-04-11
 // Main rooms list screen with bottom navigation.
 
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
+import '../../help/presentation/help_tab.dart';
+import '../../jitsi/presentation/conference_tab.dart';
 import '../../profile/presentation/profile_screen.dart';
 import '../../spaces/presentation/spaces_screen.dart';
 import 'room_tile.dart';
@@ -37,6 +39,16 @@ class _RoomsScreenState extends ConsumerState<RoomsScreen> {
       selectedIcon: Icon(Icons.person),
       label: 'Profile',
     ),
+    NavigationDestination(
+      icon: Icon(Icons.videocam_outlined),
+      selectedIcon: Icon(Icons.videocam),
+      label: 'Conference',
+    ),
+    NavigationDestination(
+      icon: Icon(Icons.help_outline),
+      selectedIcon: Icon(Icons.help),
+      label: 'Help',
+    ),
   ];
 
   Widget _buildBody() {
@@ -44,6 +56,8 @@ class _RoomsScreenState extends ConsumerState<RoomsScreen> {
       0 => const _RoomListBody(),
       1 => const SpacesScreen(embedded: true),
       2 => const ProfileScreen(embedded: true),
+      3 => const ConferenceTab(),
+      4 => const HelpTab(),
       _ => const _RoomListBody(),
     };
   }