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/shared/utils/room_preview.dart

@@ -1,4 +1,4 @@
-// Version: 1.0.0 | Created: 2026-04-02
+// Version: 1.1.0 | Created: 2026-04-02 | Updated: 2026-04-10
 // Shared utility for generating a last-message preview string from a Room.
 // Used by both RoomsRepository and SyncPersistenceService to avoid duplication.
 
@@ -12,7 +12,7 @@ String? lastMessagePreview(Room room) {
 
   return switch (lastEvent.type) {
     EventTypes.Message => lastEvent.body,
-    EventTypes.Encrypted => 'Encrypted message',
+    EventTypes.Encrypted => '\u{1F512} Encrypted — use Element to read',
     EventTypes.Sticker => 'Sticker',
     _ => null,
   };