Changed keybinding "Unbound" string to "..." for better visibility (#95142)

## About The Pull Request
A very small change that slightly improves the keybinds section by
changing the word "Unbound" to "...", also known as the
[ellipses](https://www.thepunctuationguide.com/ellipses.html).

This PR only changes string names, no other mechanics, so everything
else _should_ be working normally. Every string in the code matching
"Unbound" has been changed, including one comment for clarity.

There doesn't seem to be any issue with existing custom keybinds. All of
mine transferred seamlessly, so no one should need to redo their
keybinds.
## Why It's Good For The Game
Better visibility. Rather than playing 'find the numpad bind' scrolling
through a long list of ~100 keybinds, they show up a little easier now.

This is also the standard in other games, but not always an ellipses -
sometimes a double dash "--", but I figured there's already a dash "-"
bind by default (for the A.I. to reconnect to its shell) so the ellipses
might fit better. The ellipses is also commonly used for "awaiting
input" of some kind.


![unbound](https://github.com/user-attachments/assets/2f098852-f532-40e7-9266-6af00d60652c)
## Changelog
🆑
qol: Keybinding visual improvements: "Unbound" is grayed out and custom
binds are highlighted in green for better visibility
/🆑

code/datums/keybinding/admin.dm

@@ -97,7 +97,7 @@
 	return TRUE
 
 /datum/keybinding/admin/deadmin
-	hotkey_keys = list("Unbound")
+	hotkey_keys = list(UNBOUND_KEY)
 	name = "deadmin"
 	full_name = "Deadmin"
 	description = "Shed your admin powers"
@@ -111,7 +111,7 @@
 	return TRUE
 
 /datum/keybinding/admin/readmin
-	hotkey_keys = list("Unbound")
+	hotkey_keys = list(UNBOUND_KEY)
 	name = "readmin"
 	full_name = "Readmin"
 	description = "Regain your admin powers"

code/datums/keybinding/emote.dm

@@ -5,8 +5,8 @@
 	var/emote_key
 
 /datum/keybinding/emote/proc/link_to_emote(datum/emote/faketype)
-	hotkey_keys = list("Unbound")
-	classic_keys = list("Unbound")
+	hotkey_keys = list(UNBOUND_KEY)
+	classic_keys = list(UNBOUND_KEY)
 	emote_key = initial(faketype.key)
 	name = initial(faketype.key)
 	full_name = capitalize(initial(faketype.key))

code/datums/keybinding/living.dm

@@ -152,7 +152,7 @@
 	return TRUE
 
 /datum/keybinding/living/toggle_move_intent_alternative
-	hotkey_keys = list("Unbound")
+	hotkey_keys = list(UNBOUND_KEY)
 	name = "toggle_move_intent_alt"
 	full_name = "press to cycle move intent"
 	description = "Pressing this cycle to the opposite move intent, does not cycle back"

code/datums/keybinding/mob.dm

@@ -21,7 +21,7 @@
 	return TRUE
 
 /datum/keybinding/mob/swap_hands
-	hotkey_keys = list("Unbound")
+	hotkey_keys = list(UNBOUND_KEY)
 	name = "swap_hands"
 	full_name = "Swap hands"
 	description = ""
@@ -166,7 +166,7 @@
 	keybind_signal = COMSIG_KB_MOB_TARGETCYCLEHEAD_DOWN
 
 /datum/keybinding/mob/target/head
-	hotkey_keys = list("Unbound")
+	hotkey_keys = list(UNBOUND_KEY)
 	name = "target_head"
 	full_name = "Target: Head"
 	description = "Pressing this key targets the head. This will impact where you hit people, and can be used for surgery."