Add "Send Confirmation" feature for envelope sending

Introduced a "Send Confirmation" popup to improve user experience
when sending envelopes. The popup allows users to review recipients,
signature fields, and settings before confirming the send action.

- Added localized strings in `Resource.de-DE.resx`, `Resource.en-US.resx`,
  and `Resource.fr-FR.resx` for confirmation messages and actions.
- Updated `SenderEditorPage.razor` to include the confirmation popup
  (`DxPopup`) with body and footer templates.
- Added `RequestSendAsync` and `ConfirmSendAsync` methods to manage
  the confirmation process and sending logic.
- Replaced the old success popup logic with direct navigation to the
  sender dashboard after a successful send.
- Updated the toolbar button to trigger the confirmation popup.
- Styled the confirmation popup in `envelope-viewer.css`, including
  support for dark themes.
- Ensured proper state management for the popup and sending process.

These changes enhance usability by providing a clear confirmation
step and progress feedback during envelope sending.

EnvelopeGenerator.Application/Resources/Resource.de-DE.resx

@@ -751,6 +751,12 @@
   <data name="SenderEditor_RemoveAllSignatureFields" xml:space="preserve"><value>Alle Signaturfelder entfernen</value></data>
   <data name="SenderEditor_RemoveAllSignatureFieldsConfirm" xml:space="preserve"><value>Diese Aktion entfernt alle platzierten Signaturfelder unwiderruflich. Fortfahren?</value></data>
   <data name="SenderEditor_SelectPdf" xml:space="preserve"><value>Bitte wählen Sie eine PDF-Datei aus.</value></data>
+  <data name="SenderEditor_SendConfirm_Message" xml:space="preserve"><value>Wollen Sie den Umschlag jetzt versenden? Hinweis: Nach der Übergabe kann der Vorgang nicht mehr bearbeitet werden.</value></data>
+  <data name="SenderEditor_SendConfirm_Title" xml:space="preserve"><value>Umschlag jetzt versenden?</value></data>
+  <data name="SenderEditor_SendConfirm_Note" xml:space="preserve"><value>Bitte prüfen Sie Empfänger, Signaturfelder und Einstellungen vor dem Versenden.</value></data>
+  <data name="SenderEditor_SendConfirm_Action" xml:space="preserve"><value>Jetzt versenden</value></data>
+  <data name="SenderEditor_SendConfirm_InProgress" xml:space="preserve"><value>Umschlag wird versendet...</value></data>
+  <data name="SenderEditor_SendConfirm_InProgressHint" xml:space="preserve"><value>Bitte warten Sie einen Moment. Der Vorgang wird abgeschlossen.</value></data>
   <data name="SenderEditor_Sent_Body" xml:space="preserve"><value>Der bestehende Entwurf wurde versendet.</value></data>
   <data name="SenderEditor_Sent_Header" xml:space="preserve"><value>Umschlag gesendet</value></data>
   <data name="SenderEditor_Sent_Title" xml:space="preserve"><value>Umschlag wurde erfolgreich gesendet.</value></data>

EnvelopeGenerator.Application/Resources/Resource.en-US.resx

@@ -751,6 +751,12 @@
   <data name="SenderEditor_RemoveAllSignatureFields" xml:space="preserve"><value>Remove all signature fields</value></data>
   <data name="SenderEditor_RemoveAllSignatureFieldsConfirm" xml:space="preserve"><value>This action will permanently remove all placed signature fields. Continue?</value></data>
   <data name="SenderEditor_SelectPdf" xml:space="preserve"><value>Please select a PDF file.</value></data>
+  <data name="SenderEditor_SendConfirm_Message" xml:space="preserve"><value>Do you want to send the envelope now? Note: After handover, the process can no longer be edited.</value></data>
+  <data name="SenderEditor_SendConfirm_Title" xml:space="preserve"><value>Send envelope now?</value></data>
+  <data name="SenderEditor_SendConfirm_Note" xml:space="preserve"><value>Please review recipients, signature fields, and settings before sending.</value></data>
+  <data name="SenderEditor_SendConfirm_Action" xml:space="preserve"><value>Send now</value></data>
+  <data name="SenderEditor_SendConfirm_InProgress" xml:space="preserve"><value>Sending envelope...</value></data>
+  <data name="SenderEditor_SendConfirm_InProgressHint" xml:space="preserve"><value>Please wait a moment while we complete the process.</value></data>
   <data name="SenderEditor_Sent_Body" xml:space="preserve"><value>The existing draft has been sent.</value></data>
   <data name="SenderEditor_Sent_Header" xml:space="preserve"><value>Envelope sent</value></data>
   <data name="SenderEditor_Sent_Title" xml:space="preserve"><value>Envelope was sent successfully.</value></data>

EnvelopeGenerator.Application/Resources/Resource.fr-FR.resx

@@ -751,6 +751,12 @@
   <data name="SenderEditor_RemoveAllSignatureFields" xml:space="preserve"><value>Supprimer tous les champs de signature</value></data>
   <data name="SenderEditor_RemoveAllSignatureFieldsConfirm" xml:space="preserve"><value>Cette action supprimera définitivement tous les champs de signature placés. Continuer?</value></data>
   <data name="SenderEditor_SelectPdf" xml:space="preserve"><value>Veuillez sélectionner un fichier PDF.</value></data>
+  <data name="SenderEditor_SendConfirm_Message" xml:space="preserve"><value>Voulez-vous envoyer l'enveloppe maintenant ? Remarque : après la transmission, le processus ne peut plus être modifié.</value></data>
+  <data name="SenderEditor_SendConfirm_Title" xml:space="preserve"><value>Envoyer l'enveloppe maintenant ?</value></data>
+  <data name="SenderEditor_SendConfirm_Note" xml:space="preserve"><value>Veuillez vérifier les destinataires, les champs de signature et les paramètres avant l'envoi.</value></data>
+  <data name="SenderEditor_SendConfirm_Action" xml:space="preserve"><value>Envoyer maintenant</value></data>
+  <data name="SenderEditor_SendConfirm_InProgress" xml:space="preserve"><value>Envoi de l'enveloppe...</value></data>
+  <data name="SenderEditor_SendConfirm_InProgressHint" xml:space="preserve"><value>Veuillez patienter un instant pendant la finalisation du traitement.</value></data>
   <data name="SenderEditor_Sent_Body" xml:space="preserve"><value>Le brouillon existant a été envoyé.</value></data>
   <data name="SenderEditor_Sent_Header" xml:space="preserve"><value>Enveloppe envoyée</value></data>
   <data name="SenderEditor_Sent_Title" xml:space="preserve"><value>L'enveloppe a été envoyée avec succès.</value></data>