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/conference_tab.dart

@@ -0,0 +1,96 @@
+// Version: 1.0.0 | Created: 2026-04-10
+// Embedded conference join widget for the bottom nav tab.
+
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+/// Inline widget shown inside the RoomsScreen bottom nav.
+/// Lets authenticated users paste a Jitsi meeting link and join.
+class ConferenceTab extends StatefulWidget {
+  const ConferenceTab({super.key});
+
+  @override
+  State<ConferenceTab> createState() => _ConferenceTabState();
+}
+
+class _ConferenceTabState extends State<ConferenceTab> {
+  final _linkController = TextEditingController();
+
+  @override
+  void dispose() {
+    _linkController.dispose();
+    super.dispose();
+  }
+
+  void _joinConference() {
+    final link = _linkController.text.trim();
+    if (link.isEmpty) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(content: Text('Please paste a meeting link.')),
+      );
+      return;
+    }
+    context.push('/jitsi', extra: link);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+
+    return Center(
+      child: SingleChildScrollView(
+        padding: const EdgeInsets.symmetric(horizontal: 32, vertical: 24),
+        child: ConstrainedBox(
+          constraints: const BoxConstraints(maxWidth: 440),
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: [
+              Icon(
+                Icons.videocam_outlined,
+                size: 64,
+                color: theme.colorScheme.primary.withAlpha(180),
+              ),
+              const SizedBox(height: 16),
+              Text(
+                'Join a Conference',
+                style: theme.textTheme.titleLarge?.copyWith(
+                  fontWeight: FontWeight.bold,
+                ),
+              ),
+              const SizedBox(height: 8),
+              Text(
+                'Paste your meeting link below to join.',
+                style: theme.textTheme.bodyMedium?.copyWith(
+                  color: theme.colorScheme.onSurface.withAlpha(153),
+                ),
+              ),
+              const SizedBox(height: 32),
+              TextField(
+                controller: _linkController,
+                decoration: const InputDecoration(
+                  hintText: 'conf.m8chat.au/YourMeeting',
+                  prefixIcon: Icon(Icons.link),
+                  border: OutlineInputBorder(),
+                ),
+                textInputAction: TextInputAction.go,
+                onSubmitted: (_) => _joinConference(),
+              ),
+              const SizedBox(height: 16),
+              SizedBox(
+                width: double.infinity,
+                child: ElevatedButton.icon(
+                  onPressed: _joinConference,
+                  icon: const Icon(Icons.videocam),
+                  label: const Text('Join Meeting'),
+                  style: ElevatedButton.styleFrom(
+                    minimumSize: const Size.fromHeight(52),
+                  ),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}