Enhanced `ReportViewer.razor` with a new layout structure:
- Added `receiver-page-layout` with `receiver-signature-panel` and `receiver-viewer-wrapper` for better organization.
- Introduced a button to export signed PDFs, conditionally enabled based on `SignatureApplied`.
- Added a `DxPopup` for capturing signatures with a "Close" button.
Updated `MainLayout.razor` to remove unnecessary padding from `<article>`.
Refined `app.css`:
- Defined styles for `receiver-page-layout`, `receiver-signature-panel`, and `receiver-viewer-wrapper` to improve layout flexibility.
- Adjusted `article` to use flexbox and ensure hidden overflow.
Updated the `OnAfterRenderAsync` method in `Index.razor` to navigate to `/receiver` instead of `/reportviewer`. Removed the sidebar containing the `<NavMenu />` component in `MainLayout.razor` and adjusted the `<main>` tag to directly contain the content.
