feat(rooms): Mark as Unread (MSC2867) + Low Priority rooms

Two Matrix protocol gaps (Phase A), gate-green (683 tests):
- Mark as Unread: m.marked_unread room account data (+ com.famedly.marked_unread
  fallback), a new markedUnreadAtom binder that seeds from account data and
  clears on our own read receipt (MSC2867). RoomNavItem gains Mark as Unread /
  Read menu items and lights the row dot for a marked room. Tested.
- Low Priority: m.lowpriority room tag mirroring favourites — a context-menu
  toggle (mutually exclusive with Favorite) and a collapsed Low Priority
  category sorted to the bottom of the Home room list.

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

src/app/state/hooks/useBindAtoms.ts

@@ -3,6 +3,7 @@ import { allInvitesAtom, useBindAllInvitesAtom } from '../room-list/inviteList';
 import { allRoomsAtom, useBindAllRoomsAtom } from '../room-list/roomList';
 import { mDirectAtom, useBindMDirectAtom } from '../mDirectList';
 import { roomToUnreadAtom, useBindRoomToUnreadAtom } from '../room/roomToUnread';
+import { markedUnreadAtom, useBindMarkedUnreadAtom } from '../room/markedUnread';
 import { roomToParentsAtom, useBindRoomToParentsAtom } from '../room/roomToParents';
 import { roomIdToTypingMembersAtom, useBindRoomIdToTypingMembersAtom } from '../typingMembers';
 import { threadNotificationsAtom, useBindThreadNotificationsAtom } from '../threadNotifications';
@@ -14,6 +15,7 @@ export const useBindAtoms = (mx: MatrixClient) => {
   useBindRoomToParentsAtom(mx, roomToParentsAtom);
   useBindThreadNotificationsAtom(mx, threadNotificationsAtom);
   useBindRoomToUnreadAtom(mx, roomToUnreadAtom);
+  useBindMarkedUnreadAtom(mx, markedUnreadAtom);
 
   useBindRoomIdToTypingMembersAtom(mx, roomIdToTypingMembersAtom);
 };