feat(sender): add envelope draft save command and endpoint

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/EnvelopeService.cs

@@ -1,4 +1,5 @@
 using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.Envelopes.Commands;
 using Microsoft.AspNetCore.WebUtilities;
 using System.Net.Http;
 using System.Net.Http.Json;
@@ -94,4 +95,51 @@ public class EnvelopeService(IHttpClientFactory clientFactory)
             null,
             response.StatusCode);
     }
+
+    public async Task<SaveEnvelopeDraftResultDto?> SaveDraftAsync(
+        SaveEnvelopeDraftCommand request,
+        CancellationToken cancel = default)
+    {
+        var httpClient = clientFactory.CreateClient("EnvelopeGenerator.Server");
+        var payload = new
+        {
+            EnvelopeId = request.EnvelopeId,
+            Title = request.Title,
+            Message = request.Message,
+            TFAEnabled = request.TFAEnabled,
+            Document = request.Document is null
+                ? null
+                : new { DataAsBase64 = request.Document.DataAsBase64 },
+            Receivers = request.Receivers.Select(r => new
+            {
+                EmailAddress = r.EmailAddress,
+                Name = r.Name,
+                PhoneNumber = r.PhoneNumber,
+                Fields = r.Fields.Select(f => new { X = f.X, Y = f.Y, Page = f.Page }).ToList()
+            }).ToList()
+        };
+
+        var response = await httpClient.PostAsJsonAsync("/api/Envelope/draft", payload, _jsonOptions, cancel);
+
+        if (response.IsSuccessStatusCode)
+            return await response.Content.ReadFromJsonAsync<SaveEnvelopeDraftResultDto>(_jsonOptions, cancel);
+
+        string? message = null;
+        try
+        {
+            var error = await response.Content.ReadFromJsonAsync<ErrorResponse>(_jsonOptions, cancel);
+            message = error?.Message;
+        }
+        catch
+        {
+            message = null;
+        }
+
+        var statusCode = (int)response.StatusCode;
+        var reasonPhrase = response.ReasonPhrase ?? "Unknown error";
+        throw new HttpRequestException(
+            $"Failed to save envelope draft. Status: {statusCode} ({reasonPhrase}){(string.IsNullOrWhiteSpace(message) ? string.Empty : $" - {message}")}",
+            null,
+            response.StatusCode);
+    }
 }