Adds onion map (#4057)

If this PR fixes an issue, link it below. If not, delete these two
lines.
Resolves #4055 

## Description:

Adds a 512*512 onion map with 3 nations (Leafer Confederation, Outer
Enclave and Inner Tribe)

<img width="128" height="128" alt="thumbnail"
src="https://github.com/user-attachments/assets/8d97d8dc-6286-4e79-a459-767c936d49ec"
/>


## Please complete the following:

- [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
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

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

tktk1234567

src/core/game/Game.ts

@@ -179,6 +179,7 @@ export enum GameMapType {
   Balkans = "Balkans",
   YellowSea = "Yellow Sea",
   Labyrinth = "Labyrinth",
+  Onion = "Onion",
 }
 
 export type GameMapName = keyof typeof GameMapType;
@@ -272,6 +273,7 @@ export const mapCategories: Record<string, GameMapType[]> = {
     GameMapType.DidierFrance,
     GameMapType.Labyrinth,
     GameMapType.Sierpinski,
+    GameMapType.Onion,
   ],
   tournament: [
     GameMapType.Tourney1,

src/server/MapPlaylist.ts

@@ -107,6 +107,7 @@ const FREQUENCY: Partial<Record<GameMapName, number>> = {
   YellowSea: 5,
   Yenisei: 6,
   Labyrinth: 6,
+  Onion: 2,
 };
 
 const TEAM_WEIGHTS: { config: TeamCountConfig; weight: number }[] = [