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);
 };

src/app/state/room/markedUnread.test.ts

@@ -0,0 +1,65 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { MatrixEvent } from 'matrix-js-sdk';
+import { receiptIsMine, setMarkedUnread } from './markedUnread';
+
+// MSC2867 mark-as-unread: reading a room (our own receipt) clears the flag, so
+// `receiptIsMine` must detect only OUR receipt and ignore others'. And a write
+// must land on BOTH the stable `m.marked_unread` and the unstable
+// `com.famedly.marked_unread` key so it round-trips across servers/clients.
+
+const ME = '@me:server';
+const OTHER = '@friend:server';
+
+const receiptEvent = (content: object): MatrixEvent =>
+  ({ getContent: () => content }) as MatrixEvent;
+
+test('receiptIsMine: true when the receipt content carries our user id', () => {
+  const event = receiptEvent({
+    $abc: { 'm.read': { [ME]: { ts: 1 } } },
+  });
+  assert.equal(receiptIsMine(event, ME), true);
+});
+
+test('receiptIsMine: false when only another user has a receipt', () => {
+  const event = receiptEvent({
+    $abc: { 'm.read': { [OTHER]: { ts: 1 } } },
+  });
+  assert.equal(receiptIsMine(event, ME), false);
+});
+
+test('receiptIsMine: tolerates empty / malformed content', () => {
+  assert.equal(receiptIsMine(receiptEvent({}), ME), false);
+  assert.equal(receiptIsMine(receiptEvent({ $x: {} }), ME), false);
+});
+
+test('setMarkedUnread writes both the stable and unstable keys with the flag', async () => {
+  const calls: Array<{ type: string; content: unknown }> = [];
+  const mx = {
+    setRoomAccountData: (_roomId: string, type: string, content: unknown) => {
+      calls.push({ type, content });
+      return Promise.resolve();
+    },
+  } as any;
+
+  await setMarkedUnread(mx, '!room:server', true);
+
+  const types = calls.map((c) => c.type).sort();
+  assert.deepEqual(types, ['com.famedly.marked_unread', 'm.marked_unread']);
+  assert.ok(calls.every((c) => (c.content as { unread: boolean }).unread === true));
+});
+
+test('setMarkedUnread(false) clears both keys and does not reject if the unstable write fails', async () => {
+  const seen: string[] = [];
+  const mx = {
+    setRoomAccountData: (_roomId: string, type: string) => {
+      seen.push(type);
+      // Simulate an older server rejecting the unstable key — must not reject.
+      if (type === 'com.famedly.marked_unread') return Promise.reject(new Error('unknown type'));
+      return Promise.resolve();
+    },
+  } as any;
+
+  await assert.doesNotReject(() => setMarkedUnread(mx, '!room:server', false));
+  assert.ok(seen.includes('m.marked_unread'));
+});

src/app/state/room/markedUnread.ts

@@ -0,0 +1,97 @@
+import { atom, useSetAtom } from 'jotai';
+import { MatrixClient, MatrixEvent, Room, RoomEvent, RoomEventHandlerMap } from 'matrix-js-sdk';
+import { useEffect } from 'react';
+import { AccountDataEvent } from '../../../types/matrix/accountData';
+
+// MSC2867 — "mark a room as unread". A per-room account-data flag `{ unread }`.
+// Stable type `m.marked_unread`; servers/clients predating the stabilization use
+// the unstable `com.famedly.marked_unread`. We read either and write both so the
+// flag round-trips across the ecosystem.
+const UNSTABLE_MARKED_UNREAD = 'com.famedly.marked_unread';
+
+const readMarkedUnread = (room: Room): boolean => {
+  const stable = room.getAccountData(AccountDataEvent.MarkedUnread)?.getContent()?.unread;
+  if (typeof stable === 'boolean') return stable;
+  return room.getAccountData(UNSTABLE_MARKED_UNREAD)?.getContent()?.unread === true;
+};
+
+/** Set of room ids the user has explicitly marked as unread. */
+export const markedUnreadAtom = atom<Set<string>>(new Set<string>());
+
+/** Write (or clear) the marked-unread flag on both the stable + unstable keys. */
+export const setMarkedUnread = (
+  mx: MatrixClient,
+  roomId: string,
+  unread: boolean,
+): Promise<unknown> =>
+  Promise.all([
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    mx.setRoomAccountData(roomId, AccountDataEvent.MarkedUnread as any, { unread }),
+    // Best-effort mirror for older servers; never fail the primary write on it.
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    mx.setRoomAccountData(roomId, UNSTABLE_MARKED_UNREAD as any, { unread }).catch(() => undefined),
+  ]);
+
+export const receiptIsMine = (event: MatrixEvent, userId: string): boolean => {
+  const content = event.getContent();
+  return Object.keys(content).some((eventId) =>
+    Object.keys(content[eventId] ?? {}).some(
+      (receiptType) => content[eventId][receiptType]?.[userId],
+    ),
+  );
+};
+
+export const useBindMarkedUnreadAtom = (mx: MatrixClient, anAtom: typeof markedUnreadAtom) => {
+  const setAtom = useSetAtom(anAtom);
+
+  useEffect(() => {
+    const seed = new Set<string>();
+    mx.getRooms().forEach((room) => {
+      if (readMarkedUnread(room)) seed.add(room.roomId);
+    });
+    setAtom(seed);
+
+    const syncRoom = (room: Room) => {
+      const marked = readMarkedUnread(room);
+      setAtom((prev) => {
+        if (marked === prev.has(room.roomId)) return prev;
+        const next = new Set(prev);
+        if (marked) next.add(room.roomId);
+        else next.delete(room.roomId);
+        return next;
+      });
+    };
+
+    const onAccountData: RoomEventHandlerMap[RoomEvent.AccountData] = (_event, room) => {
+      syncRoom(room);
+    };
+    // Reading a room clears its marked-unread flag (MSC2867): when our own read
+    // receipt lands for a room that's currently marked, clear it.
+    const onReceipt: RoomEventHandlerMap[RoomEvent.Receipt] = (event, room) => {
+      const myId = mx.getUserId();
+      if (!myId || !readMarkedUnread(room)) return;
+      if (receiptIsMine(event, myId)) {
+        setMarkedUnread(mx, room.roomId, false).catch(() => undefined);
+      }
+    };
+    const onMembership: RoomEventHandlerMap[RoomEvent.MyMembership] = (room) => {
+      if (room.getMyMembership() !== 'join') {
+        setAtom((prev) => {
+          if (!prev.has(room.roomId)) return prev;
+          const next = new Set(prev);
+          next.delete(room.roomId);
+          return next;
+        });
+      }
+    };
+
+    mx.on(RoomEvent.AccountData, onAccountData);
+    mx.on(RoomEvent.Receipt, onReceipt);
+    mx.on(RoomEvent.MyMembership, onMembership);
+    return () => {
+      mx.removeListener(RoomEvent.AccountData, onAccountData);
+      mx.removeListener(RoomEvent.Receipt, onReceipt);
+      mx.removeListener(RoomEvent.MyMembership, onMembership);
+    };
+  }, [mx, setAtom]);
+};