feat(sender): route editor save and send through unified envelope upsert API

- expose POST /api/Envelope for sender-authenticated create/update/send operations

- replace client draft-specific call with EnvelopeService.UpsertAsync(CreateEnvelopeCommand)

- update sender editor save/send flow to persist draft first and send by envelope id

- allow editor hydration without document by making sender document fetch nullable

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

@@ -96,8 +96,8 @@ public class EnvelopeService(IHttpClientFactory clientFactory)
             response.StatusCode);
     }
 
-    public async Task<SaveEnvelopeDraftResultDto?> SaveDraftAsync(
-        SaveEnvelopeDraftCommand request,
+    public async Task<EnvelopeDto?> UpsertAsync(
+        CreateEnvelopeCommand request,
         CancellationToken cancel = default)
     {
         var httpClient = clientFactory.CreateClient("EnvelopeGenerator.Server");
@@ -107,6 +107,7 @@ public class EnvelopeService(IHttpClientFactory clientFactory)
             Title = request.Title,
             Message = request.Message,
             TFAEnabled = request.TFAEnabled,
+            Send = request.Send,
             Document = request.Document is null
                 ? null
                 : new { DataAsBase64 = request.Document.DataAsBase64 },
@@ -119,10 +120,10 @@ public class EnvelopeService(IHttpClientFactory clientFactory)
             }).ToList()
         };
 
-        var response = await httpClient.PostAsJsonAsync("/api/Envelope/draft", payload, _jsonOptions, cancel);
+        var response = await httpClient.PostAsJsonAsync("/api/Envelope", payload, _jsonOptions, cancel);
 
         if (response.IsSuccessStatusCode)
-            return await response.Content.ReadFromJsonAsync<SaveEnvelopeDraftResultDto>(_jsonOptions, cancel);
+            return await response.Content.ReadFromJsonAsync<EnvelopeDto>(_jsonOptions, cancel);
 
         string? message = null;
         try
@@ -138,7 +139,7 @@ public class EnvelopeService(IHttpClientFactory clientFactory)
         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}")}",
+            $"Failed to save envelope. Status: {statusCode} ({reasonPhrase}){(string.IsNullOrWhiteSpace(message) ? string.Empty : $" - {message}")}",
             null,
             response.StatusCode);
     }