Allow mappers to define additionalNations 🗺️ (#3902)

## Description:

Adds an optional `additionalNations` array to map manifests (info.json /
manifest.json), used as a name pool when a game requests more nations
than the map defines (HvN, private lobbies, solo games).

Suggested by mapmaker PatrickPlaysBadly.

When the requested nation count exceeds `nations.length`:
1. The deficit is filled by random picks from `additionalNations`
(collisions with manifest names are skipped).
2. If `additionalNations` still does not cover the deficit, the
remainder is generated procedurally as before.

Each entry supports `name`, optional `flag` and optional `coordinates`.
If `coordinates` are provided, the picked nation gets a spawn cell
(otherwise it spawns like the procedurally generated ones, with no fixed
location).

`Nation.flag` is also relaxed to optional, since many existing manifest
entries already omit it.

## 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:

FloPinguin

src/core/game/TerrainMapLoader.ts

@@ -4,6 +4,7 @@ import { GameMapLoader } from "./GameMapLoader";
 
 export type TerrainMapData = {
   nations: Nation[];
+  additionalNations: AdditionalNation[];
   gameMap: GameMap;
   miniGameMap: GameMap;
   teamGameSpawnAreas?: TeamGameSpawnAreas;
@@ -23,12 +24,22 @@ export interface MapManifest {
   map4x: MapMetadata;
   map16x: MapMetadata;
   nations: Nation[];
+  // Optional pool of fallback nation names used when a game requests more
+  // nations than the manifest defines. Picked at random; if still not enough,
+  // the remainder is generated procedurally.
+  additionalNations?: AdditionalNation[];
   teamGameSpawnAreas?: TeamGameSpawnAreas;
 }
 
 export interface Nation {
   coordinates: [number, number];
-  flag: string;
+  flag?: string;
+  name: string;
+}
+
+export interface AdditionalNation {
+  coordinates?: [number, number];
+  flag?: string;
   name: string;
 }
 
@@ -63,6 +74,14 @@ export async function loadTerrainMap(
         Math.floor(nation.coordinates[1] / 2),
       ];
     });
+    manifest.additionalNations?.forEach((nation) => {
+      if (nation.coordinates !== undefined) {
+        nation.coordinates = [
+          Math.floor(nation.coordinates[0] / 2),
+          Math.floor(nation.coordinates[1] / 2),
+        ];
+      }
+    });
   }
 
   // Scale spawn areas for compact maps
@@ -82,6 +101,7 @@ export async function loadTerrainMap(
 
   const result = {
     nations: manifest.nations,
+    additionalNations: manifest.additionalNations ?? [],
     gameMap: gameMap,
     miniGameMap: miniMap,
     teamGameSpawnAreas,