feat: Enhance terrain generation with new types and drawing mechanics

game.js

@@ -37,6 +37,13 @@ const COST_SPAWNER  = 500;
 const COST_TREE     = 50;
 const COST_SOLDIER  = 250;
 
+// Terrain costs
+const COST_WATER    = 20;
+const COST_GRASS    = 10;
+const COST_SAND     = 15;
+const COST_DIRT     = 5;
+const COST_STONE    = 25;
+
 // Earn money
 const REWARD_DELIVER_WOOD   = 5;
 const REWARD_BUILD_COMPLETE = 20;