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.Server/EnvelopeGenerator.Server.Client/Pages/SenderEditorPage.razor

@@ -318,7 +318,7 @@
                 </button>
 
                 <button class="pdf-toolbar__btn pdf-toolbar__btn--success"
-                        @onclick="SendAsync"
+                        @onclick="RequestSendAsync"
                         disabled="@_isSaving"
                         title='@L("SendEnvelope", "Umschlag senden")'>
                     @if (_isSaving)
@@ -530,6 +530,46 @@
     </BodyContentTemplate>
 </DxPopup>
 
+@* ── Send confirmation popup ── *@
+<DxPopup @bind-Visible="_sendConfirmVisible"
+         HeaderText='@L("SenderEditor_SendConfirm_Title", "Umschlag jetzt versenden?")'
+         SizeMode="SizeMode.Medium"
+         Width="460px"
+         MaxWidth="95vw"
+         ShowFooter="@(!_isSendConfirmSubmitting)"
+         ShowCloseButton="@(!_isSendConfirmSubmitting)"
+         CloseOnEscape="@(!_isSendConfirmSubmitting)"
+         CloseOnOutsideClick="false">
+    <BodyContentTemplate>
+        @if (_isSendConfirmSubmitting)
+        {
+            <div class="sender-send-progress">
+                <DxWaitIndicator Visible="true" />
+                <div>
+                    <p class="sender-send-progress__title">@L("SenderEditor_SendConfirm_InProgress", "Umschlag wird versendet...")</p>
+                    <p class="sender-send-progress__hint">@L("SenderEditor_SendConfirm_InProgressHint", "Bitte warten Sie einen Moment. Der Vorgang wird abgeschlossen.")</p>
+                </div>
+            </div>
+        }
+        else
+        {
+            <div class="sender-confirm-popup sender-confirm-popup--send">
+                <p class="sender-confirm-popup__note">
+                    @L("SenderEditor_SendConfirm_Message", "Wollen Sie den Umschlag jetzt versenden? Hinweis: Nach der Übergabe kann der Vorgang nicht mehr bearbeitet werden.")
+                </p>
+            </div>
+        }
+    </BodyContentTemplate>
+    <FooterContentTemplate>
+        <DxButton Text='@L("No", "Nein")'
+                  RenderStyle="ButtonRenderStyle.Secondary"
+                  Click='() => _sendConfirmVisible = false' />
+        <DxButton Text='@L("SenderEditor_SendConfirm_Action", "Ja, versenden")'
+                  RenderStyle="ButtonRenderStyle.Primary"
+                  Click="ConfirmSendAsync" />
+    </FooterContentTemplate>
+</DxPopup>
+
 @* ── Save result popup (success + error) ── *@
 <DxPopup @bind-Visible="_savePopupVisible"
          HeaderText='@(_saveErrorMessage is null ? _saveSuccessHeader : L("Error", "Fehler"))'
@@ -645,6 +685,8 @@
     bool _isSaving = false;
     bool _savePopupVisible = false;
     bool _clearFieldsConfirmVisible = false;
+    bool _sendConfirmVisible = false;
+    bool _isSendConfirmSubmitting = false;
     string? _saveErrorMessage = null;
     string _saveSuccessHeader = "Erfolg";
     string _saveSuccessTitle = "Vorgang erfolgreich abgeschlossen.";
@@ -1241,13 +1283,7 @@
 
             Logger.LogInformation("[SenderEditor] Envelope sent from draft. Id={Id} Status={Status}", sendResult.Id, sendResult.Status);
 
-            // Success — show popup; GoToDashboard clears cache and navigates
-            _saveErrorMessage = null;
-            _saveSuccessHeader = L("SenderEditor_Sent_Header", "Umschlag gesendet");
-            _saveSuccessTitle = L("SenderEditor_Sent_Title", "Umschlag wurde erfolgreich gesendet.");
-            _saveSuccessBody = L("SenderEditor_Sent_Body", "Der bestehende Entwurf wurde versendet.");
-            _saveNavigateToDashboard = true;
-            _savePopupVisible = true;
+            NavigationManager.NavigateTo("/sender", forceLoad: false);
         }
         catch (Exception ex)
         {
@@ -1261,6 +1297,38 @@
         }
     }
 
+    Task RequestSendAsync()
+    {
+        if (_isSaving)
+            return Task.CompletedTask;
+
+        _sendConfirmVisible = true;
+        return Task.CompletedTask;
+    }
+
+    async Task ConfirmSendAsync()
+    {
+        if (_isSendConfirmSubmitting)
+            return;
+
+        _isSendConfirmSubmitting = true;
+        await InvokeAsync(StateHasChanged);
+
+        try
+        {
+            await SendAsync();
+        }
+        finally
+        {
+            _isSendConfirmSubmitting = false;
+
+            if (_savePopupVisible)
+                _sendConfirmVisible = false;
+
+            await InvokeAsync(StateHasChanged);
+        }
+    }
+
     CreateEnvelopeCommand BuildCreateEnvelopeCommand(bool send)
     {
         NormalizeSettings();