Adds map of the USA (made for new impassable terrain feature) (#4405)

## Description:

Adds map of the continental USA. This map was made for the brand new
impassable terrain feature:
https://github.com/openfrontio/OpenFrontIO/pull/4340

Only the territory of the US is playable, with canada and mexico being
impassable terrain. Also, adds a new category called "countries" for
Country maps like this map, that use Impassable Terrain.

49 default nations (Lower 48 + D.C.) , with additional nations (native
nations and proposed states) for a total of 62, for private games and
Human Vs Nations gamemode.

Also standarizes many of the flags of the US states, since they did not
have the black border like the other flags in the game

<img width="857" height="567" alt="Captura de pantalla 2026-06-24
165158"
src="https://github.com/user-attachments/assets/70a8d760-851f-40ed-ad79-d3e210dadb90"
/>
<img width="872" height="510" alt="Captura de pantalla 2026-06-24
165510"
src="https://github.com/user-attachments/assets/c998cc10-ee89-41a7-b5e9-091be5e90da0"
/>

## 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

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

tri.star1011

src/core/game/Maps.gen.ts

@@ -92,6 +92,7 @@ export enum GameMapType {
   Tourney4 = "Tourney 8 Teams", // map-generator/assets/maps/tourney4/info.json
   TradersDream = "Traders Dream", // map-generator/assets/maps/tradersdream/info.json
   TwoLakes = "Two Lakes", // map-generator/assets/maps/twolakes/info.json
+  UnitedStates = "United States", // map-generator/assets/maps/unitedstates/info.json
   Venice = "Venice", // map-generator/assets/maps/venice/info.json
   WarshipWarship = "Warship Warship", // map-generator/assets/maps/warshipwarship/info.json
   World = "World", // map-generator/assets/maps/world/info.json
@@ -114,6 +115,7 @@ export type MapCategory =
   | "south_america"
   | "oceania"
   | "antarctica"
+  | "countries"
   | "cosmic"
   | "fictional"
   | "arcade"
@@ -132,6 +134,7 @@ export const mapCategoryOrder: readonly MapCategory[] = [
   "south_america",
   "oceania",
   "antarctica",
+  "countries",
   "cosmic",
   "fictional",
   "arcade",
@@ -802,6 +805,13 @@ export const maps: readonly MapInfo[] = [
     categories: ["europe"],
     multiplayerFrequency: 6,
   },
+  {
+    id: "UnitedStates",
+    type: GameMapType.UnitedStates,
+    translationKey: "map.unitedstates",
+    categories: ["north_america", "new", "countries"],
+    multiplayerFrequency: 9,
+  },
   {
     id: "Venice",
     type: GameMapType.Venice,