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>

pubspec.yaml

@@ -1,7 +1,7 @@
 name: m8chat_app
 description: "M8Chat — Matrix chat client for Android, iOS, and Web."
 publish_to: 'none'
-version: 1.1.0+2
+version: 1.2.0+3
 
 environment:
   sdk: '>=3.11.0 <4.0.0'
@@ -12,6 +12,7 @@ dependencies:
 
   # Matrix SDK
   matrix: ^0.33.0
+  olm: ^2.0.4
 
   # State management
   flutter_riverpod: ^3.0.0
@@ -42,6 +43,9 @@ dependencies:
   # Emoji reactions
   emoji_picker_flutter: ^4.0.0
 
+  # Web platform (required for dart:js_interop DOM access)
+  web: ^1.1.0
+
   # UI
   cached_network_image: ^3.4.0
   timeago: ^3.7.0