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/chat/presentation/message_bubble.dart

@@ -1,4 +1,4 @@
-// Version: 1.1.0 | Created: 2026-04-01 | Updated: 2026-04-02
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-10
 // Message bubble widget. Handles text, images, files, redacted, replies.
 
 import 'package:cached_network_image/cached_network_image.dart';
@@ -138,6 +138,22 @@ class _MessageContentBody extends StatelessWidget {
           fontStyle: FontStyle.italic,
         ),
       ),
+      MessageType.encrypted => Row(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          Icon(Icons.lock_outline, size: 14, color: textColour.withAlpha(153)),
+          const SizedBox(width: 6),
+          Flexible(
+            child: Text(
+              'Encrypted — use Element to read',
+              style: TextStyle(
+                color: textColour.withAlpha(153),
+                fontStyle: FontStyle.italic,
+              ),
+            ),
+          ),
+        ],
+      ),
       _ => Text(
         message.body ?? 'Unsupported message type',
         style: TextStyle(