Update popup behavior and footer button logic

Modified `<DxPopup>` properties to prevent closing via Escape,
outside clicks, or the close button. Updated the "Schliessen"
button to always be enabled, regardless of `_capturedSignature`
state.

EnvelopeGenerator.ReceiverUI/Pages/ReportViewer.razor

@@ -94,7 +94,8 @@
 HeaderText="Unterschrift erfassen"
 Width="620px"
 ShowFooter="true"
-         CloseOnEscape="true"
+CloseOnEscape="false"
+ShowCloseButton="false"
 CloseOnOutsideClick="false">
     <BodyContentTemplate>
         <ul class="nav nav-tabs mb-3">
@@ -160,7 +161,6 @@ CloseOnOutsideClick="false">
         <div class="d-flex gap-2 flex-wrap justify-content-end w-100">
             <button class="btn btn-outline-secondary" @onclick="RenewSignatureAsync">Unterschrift erneuern</button>
             <button class="btn btn-primary" @onclick="SaveSignatureAsync">Speichern</button>
-            <button class="btn btn-secondary" @onclick="CloseSignaturePopup" disabled="@(_capturedSignature is null)">Schliessen</button>
         </div>
     </FooterContentTemplate>
 </DxPopup>