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/useThreadNotifications.ts

@@ -0,0 +1,97 @@
+import { useCallback } from 'react';
+import { useAtomValue } from 'jotai';
+import { MatrixClient } from 'matrix-js-sdk';
+import { AccountDataEvent } from '../../types/matrix/accountData';
+import { threadNotificationsAtom } from '../state/threadNotifications';
+import {
+  getThreadNotificationMode,
+  pruneThreadNotifications,
+  ThreadNotificationEntry,
+  ThreadNotificationMode,
+  ThreadNotificationsContent,
+} from '../utils/threadNotifications';
+import { useMatrixClient } from './useMatrixClient';
+import { AsyncState, useAsyncCallback } from './useAsyncCallback';
+
+/** Read the current notification mode for a thread from the bound atom. */
+export function useThreadNotificationMode(
+  roomId: string,
+  threadRootId: string,
+): ThreadNotificationMode {
+  const content = useAtomValue(threadNotificationsAtom);
+  return getThreadNotificationMode(content, roomId, threadRootId);
+}
+
+const readContent = (mx: MatrixClient): ThreadNotificationsContent =>
+  ((mx as any).getAccountData(AccountDataEvent.LotusThreadNotifications)?.getContent() as
+    | ThreadNotificationsContent
+    | undefined) ?? {};
+
+const getJoinedRoomIds = (mx: MatrixClient): Set<string> => {
+  const joined = new Set<string>();
+  mx.getRooms().forEach((room) => {
+    if (room.getMyMembership() === 'join') {
+      joined.add(room.roomId);
+    }
+  });
+  return joined;
+};
+
+const writeThreadNotificationMode = async (
+  mx: MatrixClient,
+  roomId: string,
+  threadRootId: string,
+  mode: ThreadNotificationMode,
+): Promise<void> => {
+  const current = readContent(mx);
+  const now = Date.now();
+
+  // Work on a mutable clone; prune produces a fresh object so the mutations
+  // below never touch the atom's/account-data's current content.
+  const next: ThreadNotificationsContent = {
+    ...current,
+    rooms: Object.fromEntries(
+      Object.entries(current.rooms ?? {}).map(([rid, entries]) => [rid, { ...entries }]),
+    ),
+  };
+
+  const rooms = next.rooms as Record<string, Record<string, ThreadNotificationEntry>>;
+
+  if (mode === ThreadNotificationMode.Default) {
+    if (rooms[roomId]) {
+      delete rooms[roomId][threadRootId];
+      if (Object.keys(rooms[roomId]).length === 0) {
+        delete rooms[roomId];
+      }
+    }
+  } else {
+    if (!rooms[roomId]) {
+      rooms[roomId] = {};
+    }
+    rooms[roomId][threadRootId] = { mode, ts: now };
+  }
+
+  // ALWAYS prune before persisting to keep account data bounded.
+  const finalContent = pruneThreadNotifications(next, getJoinedRoomIds(mx), now);
+
+  await (mx as any).setAccountData(AccountDataEvent.LotusThreadNotifications, finalContent);
+};
+
+export function useSetThreadNotificationMode(
+  roomId: string,
+  threadRootId: string,
+): {
+  modeState: AsyncState<void, Error>;
+  setMode: (mode: ThreadNotificationMode) => Promise<void>;
+} {
+  const mx = useMatrixClient();
+
+  const [modeState, setMode] = useAsyncCallback<void, Error, [ThreadNotificationMode]>(
+    useCallback(
+      (mode: ThreadNotificationMode) => writeThreadNotificationMode(mx, roomId, threadRootId, mode),
+      [mx, roomId, threadRootId],
+    ),
+  );
+
+  return { modeState, setMode };
+}