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/features/room/thread/ThreadSummary.tsx

@@ -5,6 +5,7 @@ import { useThreadSummary } from '../../../hooks/useThreadSummary';
 import { useSetting } from '../../../state/hooks/settings';
 import { settingsAtom } from '../../../state/settings';
 import { timeDayMonthYear, timeHourMinute, today } from '../../../utils/time';
+import { ThreadNotificationMode } from '../../../utils/threadNotifications';
 
 type ThreadSummaryProps = {
   rootEvent: MatrixEvent;
@@ -12,7 +13,7 @@ type ThreadSummaryProps = {
   onOpen: (threadId: string) => void;
 };
 export function ThreadSummary({ rootEvent, room, onOpen }: ThreadSummaryProps) {
-  const { summary, unread } = useThreadSummary(rootEvent, room);
+  const { summary, unread, mode } = useThreadSummary(rootEvent, room);
   const [hour24Clock] = useSetting(settingsAtom, 'hour24Clock');
 
   if (!summary || summary.count === 0) return null;
@@ -43,6 +44,7 @@ export function ThreadSummary({ rootEvent, room, onOpen }: ThreadSummaryProps) {
           {count === 1 ? '1 reply' : `${count} replies`}
           {latestStr ? ` · ${latestStr}` : ''}
         </Text>
+        {mode === ThreadNotificationMode.Mute && <Icon size="50" src={Icons.BellMute} />}
       </Chip>
     </Box>
   );