Fixes speed potions behaving super inconsistently on magboots, duffelbags and laptops (#91415)

## About The Pull Request

The issue was threefold, first speed potions sent the comsig before
actually applying which could be abused to make items not have a
slowdown without spending it or coloring them by using it on them in a
state when they don't have a slowdown (i.e. disabled magboots). Magboots
``initial()``'d their slowdown which could break them if they somehow
got it negative, but I believe that didn't happen in actual gameplay.
And I've made duffelbags and laptops have their open slowdown set to 0
similarly to magboots when a speed potion is applied to them. Last one
may be a balance change, depending on the original intent, but I'm going
off their pre-rework behavior here (and it generally feels weird to be
the case when they get unslowdowned and then it gets reapplied when you
close and open them)

- Closes #91381

## Changelog
🆑
fix: Fixed speed potions behaving super inconsistently on magboots,
duffelbags and laptops
/🆑

code/game/objects/items/storage/dufflebags.dm

@@ -23,6 +23,7 @@
 /obj/item/storage/backpack/duffelbag/Initialize(mapload)
 	. = ..()
 	set_zipper(TRUE)
+	RegisterSignal(src, COMSIG_SPEED_POTION_APPLIED, PROC_REF(on_speed_potioned))
 
 /obj/item/storage/backpack/duffelbag/update_desc(updates)
 	. = ..()
@@ -87,11 +88,11 @@
 	zipped_up = new_zip
 	SEND_SIGNAL(src, COMSIG_DUFFEL_ZIP_CHANGE, new_zip)
 	if(zipped_up)
-		slowdown = initial(slowdown)
+		slowdown -= zip_slowdown
 		atom_storage.set_locked(STORAGE_SOFT_LOCKED)
 		atom_storage.display_contents = FALSE
 	else
-		slowdown = zip_slowdown
+		slowdown += zip_slowdown
 		atom_storage.set_locked(STORAGE_NOT_LOCKED)
 		atom_storage.display_contents = TRUE
 
@@ -99,6 +100,12 @@
 		var/mob/living/wearer = loc
 		wearer.update_equipment_speed_mods()
 
+/// Signal handler for [COMSIG_SPEED_POTION_APPLIED]. Speed potion removes the unzipped slowdown
+/obj/item/storage/backpack/duffelbag/proc/on_speed_potioned(datum/source)
+	SIGNAL_HANDLER
+	// Don't need to touch the actual slowdown here, since the speed potion does it for us
+	zip_slowdown = 0
+
 /obj/item/storage/backpack/duffelbag/cursed
 	name = "living duffel bag"
 	desc = "A cursed clown duffel bag that hungers for food of any kind. A warning label suggests that it eats food inside. \