Preference support for Guests (#72332)

## About The Pull Request

Makes it to where if you connect as a guest preferences don't error out
the ass
Resolves https://github.com/tgstation/tgstation/issues/72268
## Why It's Good For The Game

Easier local debugging, and for servers that let guests join
## Changelog
🆑
server: Preferences now support guests
/🆑

code/modules/client/preferences.dm

@@ -2,20 +2,26 @@ GLOBAL_LIST_EMPTY(preferences_datums)
 
 /datum/preferences
 	var/client/parent
-	//doohickeys for savefiles
+	/// The path to the general savefile for this datum
 	var/path
-	var/default_slot = 1 //Holder so it doesn't default to slot 1, rather the last one used
+	/// Whether or not we allow saving/loading. Used for guests, if they're enabled
+	var/load_and_save = TRUE
+	/// Ensures that we always load the last used save, QOL
+	var/default_slot = 1
+	/// The maximum number of slots we're allowed to contain
 	var/max_save_slots = 3
 
-	//non-preference stuff
-	var/muted = 0
+	/// Bitflags for communications that are muted
+	var/muted = NONE
+	/// Last IP that this client has connected from
 	var/last_ip
+	/// Last CID that this client has connected from
 	var/last_id
 
-	//game-preferences
-	var/lastchangelog = "" //Saved changlog filesize to detect if there was a change
+	/// Cached changelog size, to detect new changelogs since last join
+	var/lastchangelog = ""
 
-	//Antag preferences
+	/// List of ROLE_X that the client wants to be eligible for
 	var/list/be_special = list() //Special role selection
 
 	/// Custom keybindings. Map of keybind names to keyboard inputs.
@@ -95,13 +101,13 @@ GLOBAL_LIST_EMPTY(preferences_datums)
 		middleware += new middleware_type(src)
 
 	if(IS_CLIENT_OR_MOCK(parent))
-		if(!is_guest_key(parent.key))
-			load_path(parent.ckey)
-			if(!fexists(path))
-				try_savefile_type_migration()
-			unlock_content = !!parent.IsByondMember()
-			if(unlock_content)
-				max_save_slots = 8
+		load_and_save = !is_guest_key(parent.key)
+		load_path(parent.ckey)
+		if(load_and_save && !fexists(path))
+			try_savefile_type_migration()
+		unlock_content = !!parent.IsByondMember()
+		if(unlock_content)
+			max_save_slots = 8
 	else
 		CRASH("attempted to create a preferences datum without a client or mock!")
 	load_savefile()