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>

LOTUS_TESTING.md

@@ -675,6 +675,8 @@ Run the axe DevTools extension (or Lighthouse → Accessibility) on a room view,
 
 ## Outstanding verification backlog
 
+**Mark as Unread + Low Priority (MSC2867 / m.lowpriority, 2026-07):** Right-click a room in the sidebar → **Mark as Unread** puts a dot on the row (bold name) even with no new messages; opening/reading the room clears it, and it syncs to another device. **Mark as Read** on a marked room clears it too. Right-click → **Add to Low Priority** moves the room into a collapsed "Low Priority" category at the bottom of the room list (and removes it from Favorites if it was there, and vice-versa); **Remove from Low Priority** returns it to Rooms.
+
 **Windows rich toast (D6, 2026-07 — desktop/Windows build only):** get a message notification while the desktop app is backgrounded → the toast is attributed to **Lotus Chat** (not "PowerShell"/generic) and shows an inline **reply box + Send**; typing a reply + Send **posts it to that room**; clicking the toast body **opens the room**. Previously these silently fell back to a plain toast (no reply/click). If it still falls back, check that a `Lotus Chat.lnk` exists in the Start-Menu Programs folder.
 
 **Invite QR is now generated LOCALLY (2026-07):** Room settings → Share Room → the QR code renders (a black-on-white SVG in a white box) with **no network request** to `api.qrserver.com` (check DevTools Network — there should be no external QR fetch, and it should work offline / behind strict CSP). **Scan it** with a phone camera / Matrix app → it opens the correct `matrix.to` room-invite link. (`api.qrserver.com` was removed from the prod CSP img-src, so a regression would make the QR blank rather than silently phone home.)