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>

web/manifest.json

@@ -1,11 +1,11 @@
 {
-    "name": "m8chat_app",
-    "short_name": "m8chat_app",
+    "name": "M8Chat",
+    "short_name": "M8Chat",
     "start_url": ".",
     "display": "standalone",
-    "background_color": "#0175C2",
-    "theme_color": "#0175C2",
-    "description": "A new Flutter project.",
+    "background_color": "#0F0F1A",
+    "theme_color": "#5C35C9",
+    "description": "M8Chat — secure chat and conferencing.",
     "orientation": "portrait-primary",
     "prefer_related_applications": false,
     "icons": [