add assets and frontend to backend package

tanks-backend/TanksServer/GameLogic/CollideBullets.cs

@@ -4,7 +4,7 @@ namespace TanksServer.GameLogic;
 
 internal sealed class CollideBullets : ITickStep
 {
-    private readonly Sprite _explosiveSprite = Sprite.FromImageFile("assets/explosion.png");
+    private readonly Sprite _explosiveSprite = Sprite.FromImageFile(Path.Combine(Program.AssetsDir, "explosion.png"));
     private readonly Predicate<Bullet> _removeBulletsPredicate;
     private readonly MapEntityManager _entityManager;
     private readonly MapService _map;

tanks-backend/TanksServer/GameLogic/MapService.cs

@@ -22,9 +22,10 @@ internal sealed class MapService
 
     public MapService()
     {
-        foreach (var file in Directory.EnumerateFiles("./assets/maps/", "*.txt"))
+        var dir = Path.Combine(Program.AssetsDir, "maps");
+        foreach (var file in Directory.EnumerateFiles(dir, "*.txt"))
             LoadMapString(file);
-        foreach (var file in Directory.EnumerateFiles("./assets/maps/", "*.png"))
+        foreach (var file in Directory.EnumerateFiles(dir, "*.png"))
             LoadMapPng(file);
         Current = GetRandomMap();
     }