[small-fix 20 lines] Add FFA collusion warning (#4107)

Resolves #3900

## Description:
During the spawn phase in FFA games, display a collusion warning to
clearly communicate to new users that pre-game agreement is not allowed.
<img width="1362" height="662" alt="2026-06-01_20-31"
src="https://github.com/user-attachments/assets/bd083e91-280a-41e1-a11a-d69d5f16bc8a"
/>

- [X] I have added screenshots for all UI updates
- [X] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [X] I have added relevant tests to the test directory

## Please put your Discord username so you can be contacted if a bug or
regression is found:

goose126

---------

Co-authored-by: evanpelle <evanpelle@gmail.com>

resources/lang/en.json

@@ -1254,6 +1254,8 @@
   "heads_up_message": {
     "choose_spawn": "Choose a starting location",
     "random_spawn": "Random spawn is enabled. Selecting starting location for you...",
+    "ffa_collusion": "Reminder: teaming up before the game is not allowed in FFA",
+    "dont_show_again": "Don't show again",
     "singleplayer_game_paused": "Game paused",
     "multiplayer_game_paused": "Game paused by Lobby Creator",
     "pvp_immunity_active": "PVP immunity active for {seconds}s",