Add reboot countdown to stat panel (#88438)

## About The Pull Request

Rewrite ticker `Reboot` proc slightly to use a timer and callback for
the delay before the reboot, tracks this timer in the stat panel for
players to see. Also adds a verb to cancel a pending reboot independent
of the delay round end verb.

## Why It's Good For The Game

It's nice to be able to see exactly how much time is left until the
server restarts, and it's even nicer to see that the round end has been
delayed when the "admin has delayed round end" message gets buried in a
fast moving chat.

## Changelog

🆑
qol: The time until the server reboots is now visible in the status tab.
admin: Added a cancel reboot verb to the server tab.
/🆑

code/modules/admin/verbs/server.dm

@@ -66,6 +66,12 @@ ADMIN_VERB(restart, R_SERVER, "Reboot World", "Restarts the world immediately.",
 #undef HARDEST_RESTART
 #undef TGS_RESTART
 
+ADMIN_VERB(cancel_reboot, R_SERVER, "Cancel Reboot", "Cancels a pending world reboot.", ADMIN_CATEGORY_SERVER)
+	if(!SSticker.cancel_reboot(user))
+		return
+	log_admin("[key_name(user)] cancelled the pending world reboot.")
+	message_admins("[key_name_admin(user)] cancelled the pending world reboot.")
+
 ADMIN_VERB(end_round, R_SERVER, "End Round", "Forcibly ends the round and allows the server to restart normally.", ADMIN_CATEGORY_SERVER)
 	var/confirm = tgui_alert(user, "End the round and  restart the game world?", "End Round", list("Yes", "Cancel"))
 	if(confirm != "Yes")
@@ -131,6 +137,8 @@ ADMIN_VERB(delay_round_end, R_SERVER, "Delay Round End", "Prevent the server fro
 
 	SSticker.delay_end = TRUE
 	SSticker.admin_delay_notice = delay_reason
+	if(SSticker.reboot_timer)
+		SSticker.cancel_reboot(user)
 
 	log_admin("[key_name(user)] delayed the round end for reason: [SSticker.admin_delay_notice]")
 	message_admins("[key_name_admin(user)] delayed the round end for reason: [SSticker.admin_delay_notice]")