Add Shop system with ShopManager, ShopUI, and ShopSlotUI; implement item purchasing and shop UI functionality

Assets/Scripts/Systems/GameManager.cs

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+using System;
+using System.Collections;
+using Data;
+using Sirenix.Serialization;
+using UnityEngine;
+
+namespace Systems
+{
+    public class GameManager : MonoBehaviour
+    {
+        private float timer;
+        
+        public static GameManager Instance { get; private set; }
+
+        [OdinSerialize, SerializeField] private int currentRound = 1;
+        [OdinSerialize, SerializeField] private int coins = 0;
+        [OdinSerialize, SerializeField] private float roundTime = 60f;
+        [OdinSerialize, SerializeField] private int maxRounds = 20;
+        
+        [OdinSerialize, SerializeField] private Character player;
+        
+        public Character Player => player;
+        public int Coins => coins;
+        public int CurrentRound => currentRound;
+        public float RoundTime => roundTime;
+        public int MaxRounds => maxRounds;
+        public bool StoreIsClosed { get; set; } = true;
+
+        public event Action<int> OnRoundStart;
+        public event Action<int> OnRoundEnd;
+        public event Action OnStoreOpen;
+        
+
+        private void Awake()
+        {
+            if (Instance == null)
+            {
+                Instance = this;
+                DontDestroyOnLoad(gameObject);
+            }
+            else
+            {
+                Destroy(gameObject);
+            }
+        }
+
+        private void Start()
+        {
+            StartCoroutine(RoundLoop());
+        }
+
+        private IEnumerator RoundLoop()
+        {
+            OnStoreOpen?.Invoke();
+            yield return new WaitUntil(() => StoreIsClosed);
+            
+            for (currentRound = 1; currentRound <= maxRounds; currentRound++)
+            {
+                OnRoundStart?.Invoke(currentRound);
+                timer = roundTime;
+
+                while (timer > 0)
+                {
+                    timer -= Time.deltaTime;
+                    yield return null;
+                }
+
+                OnRoundEnd?.Invoke(currentRound);
+                StoreIsClosed = false;
+                OnStoreOpen?.Invoke();
+                yield return new WaitUntil(() => StoreIsClosed);
+            }
+        }
+        
+        public void AddCoins(int amount)
+        {
+            coins += amount;
+        }
+        
+        public void SpendCoins(int amount)
+        {
+            if (coins >= amount)
+            {
+                coins -= amount;
+            }
+        }
+    }
+}