Add Japan map (#2106)

## Description:

This PR adds a new Japan map.

<img width="975" height="967" alt="スクリーンショット 2025-09-28 18 06 11"
src="https://github.com/user-attachments/assets/5ef06c1a-f020-4109-a5df-3cba7a9a074f"
/>

## 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:

aotumuri

src/server/MapPlaylist.ts

@@ -38,6 +38,7 @@ const frequency: Partial<Record<GameMapName, number>> = {
   Halkidiki: 4,
   Iceland: 4,
   Italia: 6,
+  Japan: 6,
   Mars: 3,
   Mena: 6,
   Montreal: 6,