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/Services/EnvelopeReceiverPageDataService.cs

@@ -52,7 +52,7 @@ public class EnvelopeReceiverPageDataService(
     /// <summary>
     /// Loads the full document DTO (including receiver elements) for the authenticated sender.
     /// </summary>
-    public async Task<DocumentDto> GetDocumentDetailsOfSenderAsync(ClaimsPrincipal user, int envelopeId, CancellationToken cancellationToken = default)
+    public async Task<DocumentDto?> GetDocumentDetailsOfSenderAsync(ClaimsPrincipal user, int envelopeId, CancellationToken cancellationToken = default)
     {
         return await mediator.Send(new ReadDocumentQuery(EnvelopeId: envelopeId, UserId: user.SenderId()), cancellationToken);
     }