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/modules/modular_computers/computers/item/disks/virus_disk.dm

@@ -154,7 +154,7 @@
 		hidden_uplink.uplink_handler.generate_objectives()
 		SStraitor.register_uplink_handler(hidden_uplink.uplink_handler)
 	else
-		hidden_uplink.add_telecrystals(telecrystals)
+		hidden_uplink.uplink_handler.add_telecrystals(telecrystals)
 	telecrystals = 0
 	hidden_uplink.locked = FALSE
 	hidden_uplink.active = TRUE