Fix #4: Don't broadcast PM invite when target is Violet

- Only emit pm_invite when target_sid exists (real user)
- Violet has no socket session, so emitting to None would broadcast
  to all connected clients, leaking who is chatting with the AI

app.py

@@ -530,7 +530,7 @@ def on_pm_open(data):
     join_room(room)
     if target_sid:
         pending_pm_invites.setdefault(target_sid, set()).add(room)
-    socketio.emit("pm_invite", {"from": user["username"], "room": room}, to=target_sid)
+        socketio.emit("pm_invite", {"from": user["username"], "room": room}, to=target_sid)
     emit("pm_ready", {"with": target, "room": room})