Forces realistic animal transport. No more direct
dealer-to-barn buying — animals only travel by livestock trailer.

How it works:
- Without a trailer: the buy UI is hidden at the dealer and at your own
barn. Viewing and selling still work.
- With a trailer: vanilla behaviour. Animals go into the trailer and you
drive them home yourself.

Multiplayer-safe: the server enforces the rule via AnimalBuyEvent, so a
modified client cannot bypass the hidden UI.

Whitelist: chickens (and rabbits) are exempt by default, since the base
game has no transport trailer for them. Editable in
scripts/animalsTrailerOnly.lua.