Add sender login functionality to AuthService

Added a `SenderLoginResult` enum to represent outcomes of sender login attempts.
Implemented the `LoginSenderAsync` method in `AuthService` to authenticate sender users via the `/api/auth?cookie=true` endpoint.
The method handles HTTP response codes and returns appropriate `SenderLoginResult` values.
Included the `System.Net.Http.Json` namespace to support JSON-based HTTP requests.

EnvelopeGenerator.ReceiverUI/Services/AuthService.cs

@@ -1,4 +1,5 @@
 using System.Net;
+using System.Net.Http.Json;
 using EnvelopeGenerator.ReceiverUI.Options;
 using Microsoft.Extensions.Options;
 
@@ -6,6 +7,8 @@ namespace EnvelopeGenerator.ReceiverUI.Services;
 
 public enum EnvelopeLoginResult { Success, InvalidCode, NotFound, Error }
 
+public enum SenderLoginResult { Success, InvalidCredentials, Error }
+
 public class AuthService(HttpClient http, IOptions<ApiOptions> apiOptions)
 {
     private readonly ApiOptions _api = apiOptions.Value;
@@ -54,4 +57,25 @@ public class AuthService(HttpClient http, IOptions<ApiOptions> apiOptions)
             null, cancel);
         return response.IsSuccessStatusCode;
     }
+
+    /// <summary>
+    /// Authenticates a sender user with username and password.
+    /// Calls POST /api/auth?cookie=true with JSON body.
+    /// On success the API sets an authentication cookie automatically.
+    /// </summary>
+    public async Task<SenderLoginResult> LoginSenderAsync(string username, string password, CancellationToken cancel = default)
+    {
+        var requestBody = new { username, password };
+
+        var response = await http.PostAsJsonAsync(
+            $"{_api.BaseUrl}/api/auth?cookie=true",
+            requestBody, cancel);
+
+        return response.StatusCode switch
+        {
+            HttpStatusCode.OK => SenderLoginResult.Success,
+            HttpStatusCode.Unauthorized => SenderLoginResult.InvalidCredentials,
+            _ => SenderLoginResult.Error
+        };
+    }
 }