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/threadNotifications.ts

@@ -0,0 +1,36 @@
+import { atom, useSetAtom } from 'jotai';
+import { ClientEvent, MatrixClient, MatrixEvent } from 'matrix-js-sdk';
+import { useEffect } from 'react';
+import { AccountDataEvent } from '../../types/matrix/accountData';
+import { ThreadNotificationsContent } from '../utils/threadNotifications';
+
+// Holds the parsed `io.lotus.thread_notifications` account data. Seeded and
+// kept in sync by `useBindThreadNotificationsAtom`.
+export const threadNotificationsAtom = atom<ThreadNotificationsContent>({});
+
+const readContent = (mx: MatrixClient): ThreadNotificationsContent =>
+  ((mx as any).getAccountData(AccountDataEvent.LotusThreadNotifications)?.getContent() as
+    | ThreadNotificationsContent
+    | undefined) ?? {};
+
+export const useBindThreadNotificationsAtom = (
+  mx: MatrixClient,
+  threadNotifications: typeof threadNotificationsAtom,
+) => {
+  const setContent = useSetAtom(threadNotifications);
+
+  useEffect(() => {
+    setContent(readContent(mx));
+
+    const handleAccountData = (event: MatrixEvent) => {
+      if (event.getType() === AccountDataEvent.LotusThreadNotifications) {
+        setContent(event.getContent<ThreadNotificationsContent>() ?? {});
+      }
+    };
+
+    mx.on(ClientEvent.AccountData, handleAccountData);
+    return () => {
+      mx.removeListener(ClientEvent.AccountData, handleAccountData);
+    };
+  }, [mx, setContent]);
+};