Improve conversation update delivery

[YeungKC]

Summary

• deliver conversation snapshots and targeted deltas without Drift watches
• batch message projection, user refresh, and conversation reads to avoid per-message and per-conversation query fan-out
• warm only initial, changed, and visible conversation media instead of the full history
• consume jump-to-latest intents once and revalidate the target conversation before animation
• publish consistent mention and message-preview events
• remove obsolete paging/state wrappers, dead models, duplicate collection helpers, and unused direct dependencies

Testing

• flutter test
• flutter analyze
• flutter build macos --profile

Notes

• no database schema changes
• macOS build-generated project changes are excluded