Add LevelUpHud and UpgradeManager components; implement upgrade selection UI and item management

Assets/Scripts/Systems/UpgradeManager.cs

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+using Inventory;
+using UI;
+using UnityEngine;
+
+namespace Systems
+{
+    public class UpgradeManager : MonoBehaviour
+    {
+        private bool hasSelectedUpgrade;
+
+        [SerializeField] private InventoryManager inventoryManager;
+        [SerializeField] private LevelUpHud levelUpHud;
+
+        public bool HasSelectedUpgrade => hasSelectedUpgrade;
+        
+        public void SetHasSelectedUpgrade(bool value)
+        {
+            hasSelectedUpgrade = value;
+        }
+        
+        public void SelectUpgrade(StatModifierItem item)
+        {
+            if (hasSelectedUpgrade) return;
+
+            inventoryManager.EquipItem(item);
+            hasSelectedUpgrade = true;
+            levelUpHud.MarkAsSelected(item);
+
+        }
+
+        public void SelectUpgrade(WeaponItem weapon)
+        {
+            if (hasSelectedUpgrade) return;
+
+            inventoryManager.EquipWeapon(weapon);
+            hasSelectedUpgrade = true;
+            levelUpHud.MarkAsSelected(weapon);
+        }
+    }
+}

Assets/Scripts/Systems/UpgradeManager.cs.meta

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+fileFormatVersion: 2
+guid: e88a6318368843c3b8e02ccad05b4569
+timeCreated: 1752334253