Adds a new admin verb, Mass Modify Traits (#93767)

## About The Pull Request

Adds a verb, Mass Modify Traits, which allows an admin to apply or
remove a specific trait from all living mobs.

This lets you do things like give all mobs the `TRAIT_NOBREATHING`, or
making all mobs able to fly.

<img width="345" height="138" alt="JvkEnGMkQI"
src="https://github.com/user-attachments/assets/fad89db3-a1a5-42d7-ae54-67ca7ba92301"
/>

You can specify only cliented mobs, as well as the trait source for
removals.

<details><summary>results</summary>


![dreamseeker_K4w1Mmt73N](https://github.com/user-attachments/assets/62025b6a-1c16-4eb5-81ac-e2f5387877dd)

</details>

## Why It's Good For The Game

More tools for admins to get creative with is always a good thing, I
think! The trait system uses macros so it's difficult to use SQL to
mass-apply, so here's an easy verb for it instead.

## Changelog

🆑
admin: added new admin verb, 'Mass Modify Traits', which can be used to
add or remove a trait to/from every living mob.
/🆑

---------

Co-authored-by: Zephyr <12817816+ZephyrTFA@users.noreply.github.com>

code/modules/admin/verbs/admin.dm

@@ -125,7 +125,7 @@ ADMIN_VERB(cmd_admin_check_player_exp, R_ADMIN, "Player Playtime", "View player
 		return
 	chosen_trait = available_traits[chosen_trait]
 
-	var/source = "adminabuse"
+	var/source = TRAIT_ADMIN_GRANTED
 	switch(add_or_remove)
 		if("Add") //Not doing source choosing here intentionally to make this bit faster to use, you can always vv it.
 			if(GLOB.movement_type_trait_to_flag[chosen_trait]) //include the required element.