Refactor PDF editor with custom rendering

Replaced `DxPdfViewer` with a custom PDF rendering solution in `SenderPdfImageEditor` to improve flexibility and control. Added support for displaying placeholder messages and interactive signature fields. Introduced the `SenderPdfFieldOverlay` record to encapsulate field data.

Updated `SenderEditorPage` logic to manage page counts, image URLs, and field overlays. Added a method to estimate PDF page counts. Refactored field removal logic to use index-based operations.

Enhanced styling in `envelope-viewer.css` for improved layout and responsiveness. Updated localization files with new strings for page labels and placeholders. Adjusted JavaScript selectors in `envelope-editor.js` to support the new rendering approach.

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

@@ -735,7 +735,9 @@
   <data name="SenderEditor_LoadFileError" xml:space="preserve"><value>Fehler beim Laden der Datei: {0}</value></data>
   <data name="SenderEditor_Message_Placeholder" xml:space="preserve"><value>Bitte unterzeichnen Sie das beigefügte Dokument.</value></data>
   <data name="SenderEditor_NoDocumentLoaded" xml:space="preserve"><value>Kein Dokument geladen</value></data>
+  <data name="SenderEditor_NoPagesToDisplay" xml:space="preserve"><value>Keine Seiten zur Anzeige verfügbar.</value></data>
   <data name="SenderEditor_NoReceiversAdded" xml:space="preserve"><value>Noch keine Empfänger hinzugefügt.</value></data>
+  <data name="SenderEditor_PageLabel" xml:space="preserve"><value>Seite</value></data>
   <data name="SenderEditor_PlaceSignatureField" xml:space="preserve"><value>Signaturfeld platzieren</value></data>
   <data name="SenderEditor_RemoveAllSignatureFields" xml:space="preserve"><value>Alle Signaturfelder entfernen</value></data>
   <data name="SenderEditor_SelectPdf" xml:space="preserve"><value>Bitte wählen Sie eine PDF-Datei aus.</value></data>