Introduced drag-and-drop functionality for fields in the PDF editor:
- Added `SenderPdfFieldMoveRequest` record to encapsulate field movement data.
- Enhanced `SenderPdfImageEditor.razor` with drag-and-drop event handlers (`OnFieldDragStart`, `OnPageDropAsync`) and updated field structure.
- Updated `SenderEditorPage.razor` to handle field movement via the `OnMoveFieldAsync` method.
- Improved CSS styles for draggable fields and added a delete button with hover effects.
- Added `getDropCoordsOnPage` function in `envelope-editor.js` to calculate normalized drop coordinates.
These changes improve the user experience by enabling intuitive field placement and management in the PDF editor.
