Adds new shelves and a cash register to the commissary (#21492)

The shelves function similarly to a smartheater, but store various
things that can be found in the commissary. The current shelf types are
food & drinks, toys, cigarettes & lighters and clothing.
A cash register was also added, which can store credits and handle
transactions same way that a quik-pay can.
The quik-pay also got a small upgrade, in that it can add items to it by
clicking on the items.

Smartfridges also got an upgrade, so they can have different amounts of
overlay displays by only changing one list and one var.

| Path | Original Author | License |
| --- | --- | --- |
| icons/obj/machinery/smartfridge.dmi | Tomixcomics (Aurora Station) |
CC-BY |

<img width="586" height="458" alt="image"
src="https://github.com/user-attachments/assets/1fad54c6-b708-489c-9616-950271facc89"
/>

Example of what the shelves look like.

code/game/objects/structures/urban.dm

@@ -825,7 +825,7 @@ ABSTRACT_TYPE(/obj/structure/stairs/urban/road_ramp)
 	name = "cash compartment"
 
 /obj/structure/cash_register/mouse_drop_dragged(atom/over, mob/user, src_location, over_location, params)
-	if(user == over && ishuman(over))
+	if(user == over && ishuman(over) && storage_compartment)
 		var/mob/living/carbon/human/H = over
 		storage_compartment.open(H)