Modularize project structure by splitting responsibilities into distinct files and namespaces. Add helper methods for authentication and JSON operations.

Helper/AuthHelper.cs

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+using System;
+using System.IO;
+using System.Linq;
+using System.Net.Http;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using FsTool.Tasks;
+
+namespace FsTool.Helpers;
+
+public static class AuthHelper
+{
+    private static async Task<string?> LoadCsrfTokenAsync()
+    {
+        if (!File.Exists("csrf_token.json")) return null;
+
+        var json = await File.ReadAllTextAsync("csrf_token.json");
+        var tokenInfo = JsonSerializer.Deserialize<JsonObject>(json);
+
+        if (tokenInfo == null) return null;
+
+        var token = tokenInfo["token"]?.GetValue<string>();
+        var expiresAtString = tokenInfo["expiresAt"]?.GetValue<string>();
+
+        if (string.IsNullOrWhiteSpace(token) || string.IsNullOrWhiteSpace(expiresAtString)) return null;
+
+        if (DateTime.TryParse(expiresAtString, out var expiresAt))
+        {
+            if (DateTime.UtcNow < expiresAt.ToUniversalTime())
+                return token;
+        }
+
+        return null;
+    }
+
+    internal static async Task StoreCsrfTokenAsync(string? csrfToken, DateTime expiration)
+    {
+        if (string.IsNullOrWhiteSpace(csrfToken)) return;
+
+        var tokenInfo = new
+        {
+            token = csrfToken,
+            expiresAt = expiration
+        };
+
+        var json = JsonSerializer.Serialize(tokenInfo, new JsonSerializerOptions
+        {
+            WriteIndented = true
+        });
+
+        await File.WriteAllTextAsync("csrf_token.json", json);
+    }
+
+    public static async Task EnsureAuthenticationAsync(HttpClient httpClient)
+    {
+        // Check if header already contains a CSRF token
+        if (httpClient.DefaultRequestHeaders.TryGetValues("X-CSRF-Token", out var existingTokens))
+        {
+            var existingToken = existingTokens.FirstOrDefault();
+            if (!string.IsNullOrWhiteSpace(existingToken))
+                return;
+        }
+        
+        // Try to load CSRF token from file
+        var csrfToken = await LoadCsrfTokenAsync();
+
+        // If no valid token found, call login endpoint to get a new one
+        if (string.IsNullOrWhiteSpace(csrfToken))
+            csrfToken = await UserTasks.CallLoginEndpointAsync(httpClient);
+
+        // Set CSRF token in HTTP client headers
+        if (!string.IsNullOrWhiteSpace(csrfToken))
+        {
+            csrfToken = csrfToken.ReplaceLineEndings(string.Empty);
+
+            httpClient.DefaultRequestHeaders.Remove("X-CSRF-Token");
+            httpClient.DefaultRequestHeaders.Add("X-CSRF-Token", csrfToken);
+        }
+    }
+}

Helper/Extensions.cs

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+using System.Text;
+using System.Text.Json.Nodes;
+
+namespace FsTool.Helpers
+{
+    public static class Extensions
+    {
+        #region Public Method FsPostAsync
+
+        /// <summary>
+        ///     Sends a POST request to the specified URI with the provided JSON content.
+        /// </summary>
+        /// <param name="httpClient">The instance of <see cref="HttpClient" /> used to send the request.</param>
+        /// <param name="requestUri">The URI to which the request is sent.</param>
+        /// <param name="jsonObject">The JSON object to include in the request body.</param>
+        /// <returns>A task that represents the asynchronous operation. The task result contains the HTTP response.</returns>
+        public static async Task<HttpResponseMessage> FsPostAsync(this HttpClient httpClient, string requestUri, JsonObject jsonObject)
+        {
+            var content = new StringContent(jsonObject.ToString(), Encoding.UTF8, "application/json");
+            content.Headers.ContentType = new("application/json");
+            return await httpClient.PostAsync(requestUri, content);
+        }
+
+        #endregion
+
+        #region Public Method ToList
+
+        /// <summary>
+        ///     Converts the specified JSON node into a list of non-null <see cref="JsonNode" /> elements.
+        /// </summary>
+        /// <param name="node">The <see cref="JsonNode" /> to be converted. If null, an empty list is returned.</param>
+        /// <returns>A list of <see cref="JsonNode" /> instances containing non-null elements.</returns>
+        public static List<JsonNode> ToList(this JsonNode? node)
+        {
+            var array = node?.AsArray() ?? [];
+            return array.Where(x => x != null).Select(x => x!).ToList();
+        }
+
+        #endregion
+    }
+}