Fix NTNRC muting not working at all. (#87606)

## About The Pull Request

When fiddling around with NTNRC, I noticed that muted clients... can
still talk.
While the input bar is reddened and displays a mute message, it isn't
actually disabled- even more so, there's no sanity checks for being
muted on the `ui_act(...)`.

In this pr we just make it so the input gets disabled, and add a sanity
check to avoid people bypassing it.
## Why It's Good For The Game

...good if the app's muting feature like, actually works.
## Changelog
🆑
fix: NTNRC muting actually works again.
/🆑

code/modules/modular_computers/file_system/programs/chatroom/ntnrc_client.dm

@@ -59,6 +59,8 @@
 		if("PRG_speak")
 			if(!channel || isnull(active_channel))
 				return
+			if(src in channel.muted_clients) // Make sure we aren't muted
+				return
 			var/message = reject_bad_chattext(params["message"], MESSAGE_SIZE)
 			if(!message)
 				return