Fixed vehicle disappearance and mass/center-of-gravity calculation bugs

● Fixed severe miscalculations of vehicle mass and center of gravity. This bug caused the center of gravity to shift upward, making vehicles prone to rollover. It also made vehicle mass extremely low, causing vehicles to fly into the sky or disappear when starting up or even when touched by the player. (This fix does not affect already-broken old vehicles.)

● Fixed a bug where incorrect world parameter values were read before loading a save. Also fixed an issue where certain world parameters could not be saved.

Note: We are focusing our efforts on developing the "Merchant System", along with adding a bicycle system, hundreds of new loot items, a wide variety of furniture, various advanced wall types, and cool vehicle parts. As a result, some player feature requests or minor non-urgent bugs may be delayed. Thank you for your understanding! $