implement lobby assignment at login

src/Client.hh

@@ -156,7 +156,7 @@ public:
   std::weak_ptr<Lobby> lobby;
   uint8_t lobby_client_id = 0;
   uint8_t lobby_arrow_color = 0;
-  int64_t preferred_lobby_id = -1; // <0 = no preference
+  int64_t preferred_lobby_id = -1; // <0 = none chosen
 
   asio::steady_timer save_game_data_timer;
   asio::steady_timer send_ping_timer;