Implemented the first phase of a planned multi-format export feature for the `/sender` dashboard. Added a compact export block to the right action bar in `EnvelopeSenderPage.razor`, including a format dropdown (XLSX, CSV, PDF), a "Selected Rows Only" toggle, and an "Export" button.
The UI is fully integrated with responsive design and theme consistency across `blazing-berry`, `blazing-dark`, and `purple`. No export logic is implemented yet; placeholder handlers are in place for the export button. Styles were added to `wwwroot/css/sender-page.css` to ensure alignment with the existing design language and mobile usability.
This commit completes Phase 1 of the implementation plan. Phase 2 will add the actual export functionality.
