Add EnvelopeReceiverDto and related service class

Introduced `EnvelopeReceiverDto` and its nested models (`EnvelopeClientDto`, `EnvelopeSenderDto`, `DocumentClientDto`, `SignatureClientDto`, `ReceiverClientDto`) to represent client-side data for envelope receivers and their associated entities.

Added `EnvelopeReceiverService` to fetch `EnvelopeReceiverDto` from the API using `HttpClient`. Implemented error handling and JSON deserialization with `JsonSerializerDefaults.Web`. Updated necessary `using` directives.

EnvelopeGenerator.ReceiverUI/Services/EnvelopeReceiverService.cs

@@ -0,0 +1,27 @@
+using System.Net.Http.Json;
+using System.Text.Json;
+using EnvelopeGenerator.ReceiverUI.Models;
+using EnvelopeGenerator.ReceiverUI.Options;
+using Microsoft.Extensions.Options;
+
+namespace EnvelopeGenerator.ReceiverUI.Services;
+
+/// <summary>
+/// Retrieves the <see cref="EnvelopeReceiverDto"/> for the authenticated receiver
+/// from <c>GET api/EnvelopeReceiver/{envelopeKey}</c>.
+/// </summary>
+public class EnvelopeReceiverService(HttpClient http, IOptions<ApiOptions> apiOptions)
+{
+    private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
+
+    public async Task<EnvelopeReceiverDto?> GetAsync(string envelopeKey, CancellationToken cancel = default)
+    {
+        var url = $"{apiOptions.Value.BaseUrl}/api/EnvelopeReceiver/{Uri.EscapeDataString(envelopeKey)}";
+        var response = await http.GetAsync(url, cancel);
+
+        if (!response.IsSuccessStatusCode)
+            return null;
+
+        return await response.Content.ReadFromJsonAsync<EnvelopeReceiverDto>(_jsonOptions, cancel);
+    }
+}