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.

Changelog:
1.1.0.0
- Added: in-game settings option to enable/disable the trailer restriction.
Only the server admin can change it; the value is synced to all clients
and stored in modSettings. The loaner-trailer fix stays always on.
- Added: console command "atoToggleRestriction" (admin only).
- Fixed: contractors (Lohner) using a borrowed/shared trailer can now buy
at the dealer. The buyer's joined farm is charged, as intended (vanilla
wrongly checked the trailer owner's farm, blocking the purchase with
"must own a barn").
1.0.0.0
- Initial release: trailer-only animal purchasing.
- Server-side enforcement (multiplayer-safe).
- Chicken/rabbit whitelist.