Fix FaxBond not notifying for new faxes (#96990)

## About The Pull Request

Using a (DCS) signal was a nice idea but it ran into the problem of spam
filters on telecomms machines, if each subscribed PDA is responsible for
sending the (PDA message) signal to themselves (and because there is
always at least 5 instances of head PDAs stored in `SSwardrobe`) only
the first one gets through before telecomms refuses to send the rest.
This moves it from a DCS signal registered on the fax machine and
handled on the PDA to a system where PDAs register themselves with the
fax machine and when it receives a fax it builds a single message signal
addressed to all the targets and sends that instead.

## Why It's Good For The Game

Fixes #96902 

## Changelog
🆑
fix: fixed FaxBond not sending notifications when faxes are received
/🆑

code/modules/paperwork/fax.dm

@@ -32,6 +32,8 @@ GLOBAL_VAR_INIT(fax_autoprinting, FALSE)
 	var/allow_exotic_faxes = FALSE
 	/// This is where the dispatch and reception history for each fax is stored.
 	var/list/fax_history = list()
+	/// Lazy assoc list of (FaxBond ref string = FaxBond weakref) connected to us
+	var/list/fax_listeners
 	/// List of types which should always be allowed to be faxed
 	var/static/list/allowed_types = list(
 		/obj/item/canvas,
@@ -441,6 +443,36 @@ GLOBAL_VAR_INIT(fax_autoprinting, FALSE)
 	history_add("Receive", sender_name)
 	addtimer(CALLBACK(src, PROC_REF(vend_item), loaded), 1.9 SECONDS)
 	SEND_SIGNAL(src, COMSIG_FAX_MESSAGE_RECEIVED, sender_name)
+	if(LAZYLEN(fax_listeners))
+		alert_listeners(sender_name)
+
+/**
+ * Called when a fax is received, iterates through subscribed Faxbonds and notifies them.
+ * Arguments:
+ * * sender_name - Name of fax sender, used in the PDA message.
+*/
+/obj/machinery/fax/proc/alert_listeners(sender_name)
+	set waitfor = FALSE
+
+	var/list/targets = list()
+	for(var/refstring, weakref in fax_listeners)
+		var/datum/weakref/app_ref = weakref
+		var/datum/computer_file/program/faxbond/app = app_ref?.resolve()
+		if(!app || app.connected_faxes[fax_id]["muted"])
+			continue
+		var/datum/computer_file/program/messenger/messenger = locate() in app.computer.stored_files
+		if(messenger)
+			targets += messenger
+	if(!length(targets))
+		return
+	var/datum/signal/subspace/messaging/tablet_message/signal = new(src, list(
+		"fakename" = "Fax Notificator",
+		"fakejob" = "PDA Program",
+		"message" = "Your fax [fax_name] has received a new message from [sender_name]",
+		"targets" = targets,
+		"automated" = TRUE
+	))
+	signal.send_to_receivers()
 
 /**
  * Procedure for animating an object entering or leaving the fax machine.