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/wwwroot/css/app.css

@@ -12,6 +12,28 @@ html, body {
 
 article {
     height: calc(100vh - 70px);
+    display: flex;
+    flex-direction: column;
+    overflow: hidden;
+    padding-left: 0 !important;
+    padding-right: 0 !important;
+}
+
+.receiver-page-layout {
+    display: flex;
+    flex-direction: column;
+    height: 100%;
+    overflow: hidden;
+}
+
+.receiver-signature-panel {
+    flex: 0 0 auto;
+}
+
+.receiver-viewer-wrapper {
+    flex: 1 1 0;
+    min-height: 0;
+    overflow: hidden;
 }
 
 .valid.modified:not([type=checkbox]) {