Add desktop notification bridge for DS Chat Desktop (#49)

Offline members now also get a desktop_notification WS envelope
alongside the existing Web Push send, since Electron has no push
delivery service configured. The client only acts on it when
window.dsDesktop is present and the user's local preference allows it,
so the server needs no awareness of which clients are Electron.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/README.md

@@ -311,6 +311,40 @@ that point, so nothing online-facing is delayed, and it sidesteps
 on. An expired/invalid subscription (pywebpush 404/410) is deleted
 automatically.
 
+## Desktop notifications
+
+DS Chat Desktop (a separate Electron wrapper, not this repo) has no push
+delivery service configured, so it can't receive Web Push. Instead,
+`app/services/message_events.py`'s existing offline-member computation
+(`room members - Presence.connected_user_ids(room_id) - {sender}` — the
+same audience Web Push uses, described above) also broadcasts a
+`desktop_notification` WS envelope (`{type, id, room_id, title, body}`,
+`id` being the message's own id so the client can dedupe across
+reconnects) to every eligible offline member over their already-open
+authenticated socket, unconditionally — the server has no notion of which
+clients are running inside Electron. It's sent alongside the Web Push
+send, not instead of it, so a member with only a browser tab open is
+unaffected.
+
+The client decides whether to act on it: `frontend/src/lib/desktopBridge.ts`
+feature-detects `window.dsDesktop` (the bridge Electron's preload script
+exposes, per-method rather than via user-agent sniffing — an older wrapper
+build may be missing individual methods) and only calls
+`showNotification` when the bridge is present and the user's
+localStorage-backed preference (`ds-chat-desktop-notifications-enabled`,
+default on) allows it. This preference is deliberately a plain client-side
+flag rather than reusing `PushSubscription` — desktop notifications need
+no server round trip to enable/disable, unlike a push subscription which
+has a row to create/delete. `frontend/src/components/DesktopNotificationBridge.tsx`
+is mounted once, as a sibling of the routed pages inside the `user.id`-keyed
+`ChatSocketProvider`, so it subscribes exactly once per authenticated
+session; it also wires `window.dsDesktop.onNotificationClick` to navigate
+to the notification's room.
+
+No `User`/`PushSubscription` schema change was needed for this feature —
+the only backend change is the new `desktop_notification` envelope type,
+covered by `backend/tests/test_desktop_notifications.py`.
+
 ## Room roles and membership (Phase 2)
 
 Rooms can be `open` (anyone can join via `POST /api/rooms/{id}/join`) or