Use ConcurrentHashMap for kit storage in KitManager

- Replace `HashMap` with `ConcurrentHashMap` for `kitByKitIDMap`
- Generalize field type to `Map<String, ItemStack[]>` for cleaner abstraction

Author: rossnoah

src/main/java/dev/noah/perplayerkit/KitManager.java

@@ -31,19 +31,20 @@
 
 import java.io.IOException;
 import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
 
 public class KitManager {
     private static KitManager instance;
     private final PerPlayerKit plugin;
-    private final HashMap<String, ItemStack[]> kitByKitIDMap;
+    private final Map<String, ItemStack[]> kitByKitIDMap;
     private final HashMap<UUID, Integer> lastKitUsedByPlayer;
     private final List<PublicKit> publicKitList;
 
     public KitManager(PerPlayerKit plugin) {
         this.plugin = plugin;
         lastKitUsedByPlayer = new HashMap<>();
         publicKitList = new ArrayList<>();
-        kitByKitIDMap = new HashMap<>();
+        kitByKitIDMap = new ConcurrentHashMap<>();
         instance = this;
     }
 
@@ -544,4 +545,4 @@ private void applyKitLoadEffects(Player player, boolean isEnderChest) {
             }
         }
     }
-}
+}