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

map-generator/assets/maps/japan/info.json

@@ -0,0 +1,77 @@
+{
+  "name": "japan",
+  "nations": [
+    {
+      "coordinates": [1895, 288],
+      "name": "Hokkaido",
+      "strength": 2,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [1647, 897],
+      "name": "Tohoku",
+      "strength": 2,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [1613, 1133],
+      "name": "Kanto",
+      "strength": 2,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [1391, 1194],
+      "name": "Chubu",
+      "strength": 2,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [1191, 1370],
+      "name": "Kansai",
+      "strength": 2,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [932, 1308],
+      "name": "Chugoku",
+      "strength": 2,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [949, 1454],
+      "name": "Shikoku",
+      "strength": 2,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [709, 1570],
+      "name": "Kyushu",
+      "strength": 2,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [397, 2283],
+      "name": "Okinawa",
+      "strength": 1,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [1493, 1246],
+      "name": "Mount Fuji",
+      "strength": 1,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [1565, 1178],
+      "name": "Tokyo",
+      "strength": 3,
+      "flag": "jp"
+    },
+    {
+      "coordinates": [1158, 1290],
+      "name": "Kyoto",
+      "strength": 3,
+      "flag": "jp"
+    }
+  ]
+}