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/ws/focus_presence.py

@@ -0,0 +1,45 @@
+import uuid
+
+from redis.asyncio import Redis
+
+
+class FocusPresence:
+    """Cross-instance "is this desktop-mode user's window currently
+    unfocused," used only to widen desktop_notification/push eligibility
+    beyond plain room-connection state (#59). A Redis hash (field =
+    user_id, value = refcount of that user's currently-blurred desktop
+    connections), parallel to Presence/GlobalPresence.
+
+    Absence from this hash is the default and means "focused." That default
+    is also exactly right for every browser-tab connection: only the
+    desktop client ever sends focus/blur frames at all (see chat.py's
+    "focus" envelope handling), so a browser user never appears here --
+    their attention is already fully captured by Presence's room-connection
+    state, which stays visibility-gated with no separate focus signal.
+
+    Refcounted for the same multi-connection reason as Presence/
+    GlobalPresence, with the same known simplification: two desktop windows
+    for one user, one focused and one blurred, count as "unfocused" here
+    (refcount > 0) even though the user does have attention somewhere. That
+    errs toward notifying rather than silently missing one, which is the
+    safer failure mode for a notification.
+    """
+
+    def __init__(self, redis: Redis) -> None:
+        self._redis = redis
+
+    def _key(self) -> str:
+        return "presence:unfocused"
+
+    async def mark_blurred(self, user_id: uuid.UUID) -> None:
+        await self._redis.hincrby(self._key(), str(user_id), 1)
+
+    async def mark_focused(self, user_id: uuid.UUID) -> None:
+        key = self._key()
+        field = str(user_id)
+        remaining = await self._redis.hincrby(key, field, -1)
+        if remaining <= 0:
+            await self._redis.hdel(key, field)
+
+    async def is_unfocused(self, user_id: uuid.UUID) -> bool:
+        return await self._redis.hexists(self._key(), str(user_id))