Introduced a new export feature to the EnvelopeSenderPage, including a dropdown for selecting export formats (XLSX, CSV, PDF) and a checkbox to export only selected rows. Added a placeholder export button ("Exportieren") that is currently disabled, with export logic to be implemented in a future phase.
Added private fields `_selectedExportFormat` and `_exportSelectedRowsOnly` to manage the export state. Implemented `IsExportDisabled()` to control button availability and a placeholder method `ExportSelectedFormatPlaceholderAsync()`.
Updated `sender-page.css` to style the export feature, including the export button, dropdown, and checkbox. Added theme-specific styles for "purple" and "blazing-dark" themes. Adjusted toolbar element order to position the export group before the logout button.