Add signature panel and layout updates for ReportViewer

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.

EnvelopeGenerator.ReceiverUI/Shared/MainLayout.razor

@@ -7,7 +7,7 @@
             <a href="https://docs.microsoft.com/aspnet/" target="_blank">About</a>
         </div>
 
-        <article class="content px-4">
+        <article class="content">
             @Body
         </article>
     </main>