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.