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/main.py

@@ -27,6 +27,7 @@ from app.routers import (
 from app.ws.broadcaster import Broadcaster
 from app.ws.chat import router as ws_router
 from app.ws.connection_manager import ConnectionManager
+from app.ws.focus_presence import FocusPresence
 from app.ws.global_presence import GlobalPresence
 from app.ws.presence import Presence
 
@@ -82,6 +83,7 @@ def create_app() -> FastAPI:
     app.state.redis = Redis.from_url(settings.redis_url, decode_responses=True)
     app.state.presence = Presence(app.state.redis)
     app.state.global_presence = GlobalPresence(app.state.redis)
+    app.state.focus_presence = FocusPresence(app.state.redis)
     app.state.broadcaster = Broadcaster(app.state.redis, app.state.connection_manager)
 
     app.include_router(health.router)