Adds a signal to buying items from the uplink (& fixes TC misinfo) (#81372)

## About The Pull Request

Adds a signal when someone buys an item from the uplink and removes
single-letter vars from the ``spawn_item`` proc, and adds/standardizes
add/removing of telecrystals from uplinks (and admin setting how much TC
they have) to ensure the UI always has the right amount of telecrystals
displayed in it.

## Why It's Good For The Game

There are reasons why someone would want to hook up to a traitor's
uplink and listen to items they purchase to do any special effect
on-purchase, so this adds support to do anything in the future with it.
Also tells players how much TC they actually have without forcing them
to close/reopen the UI every time they insert some TC in it by hand.

## Changelog

🆑
fix: Uplinks now update their UI when you add telecrystals in them, so
you don't need to close and reopen it.
/🆑

code/datums/mind/_mind.dm

@@ -457,9 +457,14 @@
 				if(check_rights(R_FUN))
 					var/datum/component/uplink/U = find_syndicate_uplink()
 					if(U)
-						var/crystals = input("Amount of telecrystals for [key]","Syndicate uplink", U.uplink_handler.telecrystals) as null | num
-						if(!isnull(crystals))
-							U.uplink_handler.telecrystals = crystals
+						var/crystals = tgui_input_number(
+							user = usr,
+							message = "Amount of telecrystals for [key]",
+							title = "Syndicate uplink",
+							default = U.uplink_handler.telecrystals,
+							)
+						if(crystals && isnum(crystals))
+							U.uplink_handler.set_telecrystals(crystals)
 							message_admins("[key_name_admin(usr)] changed [current]'s telecrystal count to [crystals].")
 							log_admin("[key_name(usr)] changed [current]'s telecrystal count to [crystals].")
 			if("progression")