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/core/config/app_config.dart

@@ -1,9 +1,6 @@
-// Version: 1.0.0 | Created: 2026-04-01
+// Version: 2.0.0 | Created: 2026-04-01 | Updated: 2026-04-11
 // App-wide constants. Change matrixBaseUrl for different environments.
 
-/// Central configuration for the M8Chat application.
-/// All server URLs and app-level constants live here — never scatter
-/// magic strings through feature code.
 abstract final class AppConfig {
   static const String matrixBaseUrl = 'https://matrix.m8chat.au';
   static const String matrixServerName = 'matrix.m8chat.au';
@@ -14,11 +11,8 @@ abstract final class AppConfig {
     'turns:matrix.m8chat.au:5349',
   ];
   static const String appName = 'M8Chat';
-  static const String appVersion = '1.0.0';
+  static const String appVersion = '1.3.0';
 
-  // Secure storage key names
-  static const String storageKeyAccessToken = 'access_token';
-  static const String storageKeyUserId = 'user_id';
-  static const String storageKeyDeviceId = 'device_id';
-  static const String storageKeyHomeserver = 'homeserver';
+  // Jitsi conferencing
+  static const String jitsiDomain = 'conf.m8chat.au';
 }