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/types/matrix/accountData.ts

@@ -2,6 +2,8 @@ export enum AccountDataEvent {
   PushRules = 'm.push_rules',
   Direct = 'm.direct',
   IgnoredUserList = 'm.ignored_user_list',
+  // [MSC2867] Per-room "mark as unread" flag (room account data).
+  MarkedUnread = 'm.marked_unread',
 
   CinnySpaces = 'in.cinny.spaces',