docs: Slack-style per-thread notifications (P4-1) across catalog/README/TODO/BUGS

LOTUS_FEATURES: Notifications subsection under Threads (participating default,
per-thread All/Mentions/Mute, badge behavior). README: thread-notifications
bullet. LOTUS_TODO: P4-1 → [~] + 6-step live-QA checklist + caveats.
LOTUS_BUGS: verification row.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -19,6 +19,7 @@ The Lotus Chat logo (`public/res/Lotus.png`) is a derivative work based on the o
 ### Messaging
 
 - Threads: reply in a thread and read/write the whole conversation in a side panel — root messages show a "N replies" chip with an unread badge (threaded replies live in the panel now, not inline in the room)
+- Slack-style thread notifications: by default you're only pinged for threads you're in or where you're @mentioned; set any thread to All / Mentions-only / Mute from the panel's bell menu (muted threads stop bumping badges; syncs across devices)
 - See who has read each message, and track delivery status (sending / sent / failed)
 - Bookmark any message and revisit saved messages from the sidebar
 - Schedule messages to send at a specific time