fix negative cash heal bug

src/cash_heal.rs

@@ -18,7 +18,13 @@ fn on_heal_trigger(
         return;
     }
 
+    let cost = 10;
+
+    if cash.cash < cost {
+        return;
+    }
+
     hp.set_health(100);
 
-    cash.cash = cash.cash.saturating_sub(10);
+    cash.cash = cash.cash.saturating_sub(cost);
 }