Fix desktop messages not appearing live until refocus (#59)

Desktop mode's focus gating (from #49) made losing OS focus send "leave"
for every open room, which stopped live message delivery to that room,
not just notification eligibility -- so a message wouldn't render until
the room was manually left and rejoined. Room join/leave is now gated on
visibility alone, matching the browser; notification eligibility gets its
own separate signal (a "focus"/"blur" WS frame tracked by a new
Redis-backed FocusPresence), so a connected-but-unfocused desktop member
still gets notified without losing live delivery.

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

backend/app/routers/webhooks.py

@@ -25,4 +25,5 @@ async def incoming_webhook_endpoint(
 
     broadcaster = request.app.state.broadcaster
     presence = request.app.state.presence
-    await broadcast_new_message(db, broadcaster, presence, room.id, message, sender)
+    focus_presence = request.app.state.focus_presence
+    await broadcast_new_message(db, broadcaster, presence, focus_presence, room.id, message, sender)