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/state/hooks/useBindAtoms.ts

@@ -5,12 +5,14 @@ import { mDirectAtom, useBindMDirectAtom } from '../mDirectList';
 import { roomToUnreadAtom, useBindRoomToUnreadAtom } from '../room/roomToUnread';
 import { roomToParentsAtom, useBindRoomToParentsAtom } from '../room/roomToParents';
 import { roomIdToTypingMembersAtom, useBindRoomIdToTypingMembersAtom } from '../typingMembers';
+import { threadNotificationsAtom, useBindThreadNotificationsAtom } from '../threadNotifications';
 
 export const useBindAtoms = (mx: MatrixClient) => {
   useBindMDirectAtom(mx, mDirectAtom);
   useBindAllInvitesAtom(mx, allInvitesAtom);
   useBindAllRoomsAtom(mx, allRoomsAtom);
   useBindRoomToParentsAtom(mx, roomToParentsAtom);
+  useBindThreadNotificationsAtom(mx, threadNotificationsAtom);
   useBindRoomToUnreadAtom(mx, roomToUnreadAtom);
 
   useBindRoomIdToTypingMembersAtom(mx, roomIdToTypingMembersAtom);