Integrate mentions into the master messenger UI

[cassi-volkova]

Summary

• add the Figma-aligned My Activity screen and five-destination application shell on top of the existing master navigation
• load real mentioned messages and project unread mentions into both stream and topic @ badges
• add a Mentions timeline that resolves authors, streams, and topics and opens the matching conversation
• keep Activity and Chat on one shared ChatViewModel/EventsRepository lifecycle

Verification

• ./gradlew test
• ./gradlew lintExordosDebug
• ./gradlew clean assembleExordosDebug
• installed the Exordos debug build on a physical Pixel 5 running Android 14
• verified real unread mention count, Mentions timeline content, stream @, topic @, and opening the matching conversation from a mention
• checked runtime logs for application crashes; none observed

The device screenshots contain live Workspace content, so they are being shared only in the originating Workspace topic.

[cassi-volkova]

Self-review is complete. The review caught a lifecycle problem in the first port: Activity and Chat could create separate ChatViewModel instances and therefore duplicate catalog loading and WebSocket setup. Both top-level routes now share one instance and one EventsRepository lifecycle. I also removed the screenshot fixture, rechecked owner-scoped mention replacement, stream/topic unread projection, message-to-conversation navigation, and the final master diff. I found no remaining issues after the unit, lint, clean-build, and physical-device passes.