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/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'));
+});