ChemMaster QoL: Reusable Custom Transfer Amounts (#96563)

## About The Pull Request


https://github.com/user-attachments/assets/a6c6d3d9-3284-4a45-80c4-14dbc4956cf8

Adds custom transfer amount functionality to the ChemMaster UI through a
number input (by the eject button) and paired button.

## Why It's Good For The Game

If I have to hit the one-off custom transfer button, several times in a
row, for my artisanal chemical atrocities one more time, I'm going to
lose it.

Less-jokingly, maybe someone has some oddly specific mixing things they
need it for (I know I do!).

code/modules/reagents/chemistry/machinery/chem_master.dm

@@ -32,6 +32,8 @@
 	var/printing_speed = 0.75 SECONDS
 	/// Amount of layers which printed pills will be coated in
 	var/pill_layers = 3
+	/// Current custom transfer amount
+	var/custom_transfer_amount = 15
 
 /obj/machinery/chem_master/Initialize(mapload)
 	create_reagents(100)
@@ -281,6 +283,7 @@
 	.["selectedPillDuration"] = pill_layers
 	var/obj/item/held_item = user.get_active_held_item()
 	.["hasBeakerInHand"] = held_item?.is_chem_container() || FALSE
+	.["customTransferAmount"] = custom_transfer_amount
 
 	//contents of source beaker
 	var/list/beaker_data = null
@@ -414,6 +417,18 @@
 
 			return TRUE
 
+		if("setCustomTransfer")
+			var/target = params["target"]
+			if(isnull(target))
+				return FALSE
+
+			target = text2num(target)
+			if(isnull(target))
+				return FALSE
+
+			custom_transfer_amount = clamp(target, 0, beaker.volume)
+			return TRUE
+
 		if("transfer")
 			if(is_printing)
 				say("The buffer is locked while printing.")