Refactor SignatureService to DocReceiverElementService

Renamed the `SignatureService` class to `DocReceiverElementService` to reflect its updated purpose. Updated the `GetAsync` method to use the `DocReceiverElement` API endpoint (`/api/DocReceiverElement/{envelopeKey}`) instead of the `Signature` API endpoint. These changes align the service with the new `DocReceiverElement` context.

EnvelopeGenerator.ReceiverUI/Services/DocReceiverElementService.cs

@@ -0,0 +1,24 @@
+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;
+
+public class DocReceiverElementService(HttpClient http, IOptions<ApiOptions> apiOptions)
+{
+    private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
+
+    public async Task<IReadOnlyList<SignatureDto>> GetAsync(string envelopeKey, CancellationToken cancel = default)
+    {
+        var url = $"{apiOptions.Value.BaseUrl}/api/DocReceiverElement/{Uri.EscapeDataString(envelopeKey)}";
+        var response = await http.GetAsync(url, cancel);
+
+        if (!response.IsSuccessStatusCode)
+            throw new HttpRequestException($"Failed to retrieve signatures for envelope {envelopeKey}: {response.StatusCode} {response.ReasonPhrase}");
+
+        var result = await response.Content.ReadFromJsonAsync<List<SignatureDto>>(_jsonOptions, cancel);
+        return result ?? [];
+    }
+}