Add login route and enhance login page functionality

Added a new `/login/{**catch-all}` route in `yarp.json` for the `receiver-ui` cluster. Updated `Login.razor` with a modernized UI, including a gradient header, improved error handling, and a toggle for password visibility. Integrated `AuthService` for API-based login validation.

Refactored `Login.razor` logic to handle login outcomes (`Success`, `InvalidCode`, `NotFound`, `Error`) using the new `EnvelopeLoginResult` enum. Added `LoginEnvelopeReceiverAsync` method in `AuthService` to handle login requests via `POST /api/Auth/envelope-receiver/{key}`. Improved code maintainability and user experience.

EnvelopeGenerator.ReceiverUI/Services/AuthService.cs

@@ -4,6 +4,8 @@ using Microsoft.Extensions.Options;
 
 namespace EnvelopeGenerator.ReceiverUI.Services;
 
+public enum EnvelopeLoginResult { Success, InvalidCode, NotFound, Error }
+
 public class AuthService(HttpClient http, IOptions<ApiOptions> apiOptions)
 {
     private readonly ApiOptions _api = apiOptions.Value;
@@ -17,4 +19,27 @@ public class AuthService(HttpClient http, IOptions<ApiOptions> apiOptions)
         var response = await http.GetAsync($"{_api.BaseUrl}/api/auth/check/envelope/{Uri.EscapeDataString(envelopeKey)}", cancel);
         return response.StatusCode == HttpStatusCode.OK;
     }
+
+    /// <summary>
+    /// Submits the access code for the given envelope key.
+    /// Calls POST /api/Auth/envelope-receiver/{key} with multipart/form-data.
+    /// On success the API sets an authentication cookie automatically.
+    /// </summary>
+    public async Task<EnvelopeLoginResult> LoginEnvelopeReceiverAsync(string envelopeKey, string accessCode, CancellationToken cancel = default)
+    {
+        var form = new MultipartFormDataContent();
+        form.Add(new StringContent(accessCode), "AccessCode");
+
+        var response = await http.PostAsync(
+            $"{_api.BaseUrl}/api/Auth/envelope-receiver/{Uri.EscapeDataString(envelopeKey)}",
+            form, cancel);
+
+        return response.StatusCode switch
+        {
+            HttpStatusCode.OK => EnvelopeLoginResult.Success,
+            HttpStatusCode.Unauthorized => EnvelopeLoginResult.InvalidCode,
+            HttpStatusCode.NotFound => EnvelopeLoginResult.NotFound,
+            _ => EnvelopeLoginResult.Error
+        };
+    }
 }