Implement AuthService and ApiClient core logic

Core/ApiClient.cs

@@ -0,0 +1,36 @@
+using Newtonsoft.Json;
+using System;
+using System.Net.Http;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Chtn.CSharp.SDK.Core
+{
+    public class ApiClient
+    {
+        private readonly HttpClient _httpClient;
+        private readonly string _baseUrl;
+
+        public ApiClient(string baseUrl)
+        {
+            _baseUrl = baseUrl.EndsWith("/") ? baseUrl : baseUrl + "/";
+            _httpClient = new HttpClient();
+        }
+
+        public async Task<TResponse> PostAsync<TRequest, TResponse>(string endpoint, TRequest data)
+        {
+            var json = JsonConvert.SerializeObject(data);
+            var content = new StringContent(json, Encoding.UTF8, "application/json");
+
+            var response = await _httpClient.PostAsync(_baseUrl + endpoint, content);
+            var responseJson = await response.Content.ReadAsStringAsync();
+
+            if (!response.IsSuccessStatusCode)
+            {
+                throw new Exception($"API Hiba ({response.StatusCode}): {responseJson}");
+            }
+
+            return JsonConvert.DeserializeObject<TResponse>(responseJson);
+        }
+    }
+}