feat(threads): Slack-style per-thread notifications (P4-1)

Default = Participating: thread replies notify only when you've posted in the
thread or are @mentioned; per-thread override to All / Mentions-only / Mute via
a bell menu in the thread panel header. Modes sync across devices in
io.lotus.thread_notifications account data (pruned on write: left rooms, >180d,
cap 200/room). Muted threads: no notifications/sounds, chip badge suppressed
(+BellMute glyph), and their counts are subtracted from the room's sidebar
badge (client-side; clamped ≥0).

Also fixes the thread notification path itself: thread replies are now owned by
exactly ONE handler (room-level ThreadEvent.NewReply via a new useRoomsListener
hook, with per-thread dedupe, panel-aware focus suppression, and per-thread OS
tag coalescing) — the existing RoomEvent.Timeline handlers in the notifier and
the unread binder are explicitly thread-guarded, eliminating the previously
un-gated/double path. Room badges now also refresh live on
RoomEvent.UnreadNotifications (surgical per-room PUT; fixes thread-badge lag).

Pure decision core shouldNotifyThreadReply (13-case matrix) + prune + unread
subtraction: +32 tests (648 total). E2EE caveat documented: mentions-only may
under-notify pre-decryption (same class as the existing path).

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

src/app/hooks/useThreadSummary.ts

@@ -1,4 +1,5 @@
 import { useEffect, useState } from 'react';
+import { useAtomValue } from 'jotai';
 import {
   MatrixEvent,
   NotificationCountType,
@@ -8,6 +9,8 @@ import {
   ThreadEvent,
 } from 'matrix-js-sdk';
 import { getThreadSummary, ThreadSummaryData } from '../features/room/thread/threadSummary';
+import { threadNotificationsAtom } from '../state/threadNotifications';
+import { getThreadNotificationMode, ThreadNotificationMode } from '../utils/threadNotifications';
 
 /**
  * Reactive thread summary + unread count for a root event's "N replies" chip.
@@ -18,9 +21,14 @@ import { getThreadSummary, ThreadSummaryData } from '../features/room/thread/thr
 export const useThreadSummary = (
   rootEvent: MatrixEvent,
   room: Room,
-): { summary: ThreadSummaryData | undefined; unread: number } => {
+): { summary: ThreadSummaryData | undefined; unread: number; mode: ThreadNotificationMode } => {
   const threadId = rootEvent.getId();
 
+  const threadNotifications = useAtomValue(threadNotificationsAtom);
+  const mode = threadId
+    ? getThreadNotificationMode(threadNotifications, room.roomId, threadId)
+    : ThreadNotificationMode.Default;
+
   const [summary, setSummary] = useState<ThreadSummaryData | undefined>(() =>
     getThreadSummary(rootEvent),
   );
@@ -53,5 +61,7 @@ export const useThreadSummary = (
     };
   }, [rootEvent, room, threadId]);
 
-  return { summary, unread };
+  const muted = mode === ThreadNotificationMode.Mute;
+
+  return { summary, unread: muted ? 0 : unread, mode };
 };