Add Japan as map (#384)

## Description:
There are fewer bots on the mainland, so taking control there can make
you more powerful.
To balance that, no rivers have been placed there, meaning you won’t
gain trade advantages from them.
The battlefield is split into two fronts, allowing for exciting
amphibious assaults with nuclear warfare.

I'm not sure about the white line on the map, but I don't think it's a
problem.

<img width="382" alt="スクリーンショット 2025-03-31 21 21 26"
src="https://github.com/user-attachments/assets/e1428ad8-febc-4eee-9c73-f27dd6fc207a"
/>
<img width="541" alt="スクリーンショット 2025-03-31 21 22 01"
src="https://github.com/user-attachments/assets/e2e63664-dde6-4d0b-a9d3-4f759d8924c6"
/>
<img width="441" alt="スクリーンショット 2025-03-31 21 22 15"
src="https://github.com/user-attachments/assets/eb386896-ed02-463b-ba2e-1b991dba8acb"
/>

![スクリーンショット 2025-03-31 21 23
20](https://github.com/user-attachments/assets/aa7685a6-74d7-4d89-9595-fe1731b43162)


## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

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

<DISCORD USERNAME>
aotumuri

src/client/components/Maps.ts

@@ -20,6 +20,7 @@ export const MapDescription: Record<keyof typeof GameMapType, string> = {
   GatewayToTheAtlantic: "Gateway to the Atlantic",
   Australia: "Australia",
   Iceland: "Iceland",
+  Japan: "Japan",
 };
 
 @customElement("map-display")

src/client/utilities/Maps.ts

@@ -13,6 +13,7 @@ import britannia from "../../../resources/maps/BritanniaThumb.webp";
 import gatewayToTheAtlantic from "../../../resources/maps/GatewayToTheAtlanticThumb.webp";
 import australia from "../../../resources/maps/AustraliaThumb.webp";
 import iceland from "../../../resources/maps/IcelandThumb.webp";
+import japan from "../../../resources/maps/JapanThumb.webp";
 
 import { GameMapType } from "../../core/game/Game";
 
@@ -48,6 +49,8 @@ export function getMapsImage(map: GameMapType): string {
       return australia;
     case GameMapType.Iceland:
       return iceland;
+    case GameMapType.Japan:
+      return japan;
     default:
       return "";
   }

src/core/game/Game.ts

@@ -60,6 +60,7 @@ export enum GameMapType {
   GatewayToTheAtlantic = "Gateway to the Atlantic",
   Australia = "Australia",
   Iceland = "Iceland",
+  Japan = "Japan",
 }
 
 export enum GameType {

src/core/game/TerrainMapFileLoader.ts

@@ -39,6 +39,7 @@ const MAP_FILE_NAMES: Record<GameMapType, string> = {
   [GameMapType.GatewayToTheAtlantic]: "GatewayToTheAtlantic",
   [GameMapType.Australia]: "Australia",
   [GameMapType.Iceland]: "Iceland",
+  [GameMapType.Japan]: "Japan",
 };
 
 class GameMapLoader {