Enhance PDF viewer and add embed page with file upload

- Added CSS styles for `.pdf-viewer` and its child elements
  to ensure proper dimensions and layout for PDF display.
- Enhanced `EnvelopeReceiverPage_DxPdfViewer.razor` with
  conditional rendering for improved user feedback.
- Introduced `EnvelopeReceiverPage_embed.razor` with a new
  route `/envelope/Embed`, drag-and-drop file upload, and
  embedded PDF viewer using `<embed>`.
- Implemented default PDF loading from embedded resources
  and Base64 encoding for embedding.
- Refactored file upload handling with `OnFilesUploading`
  and centralized allowed file types logic.
- Improved user experience with success and informational
  messages for file upload and PDF viewing.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeReceiverPage_DxPdfViewer.razor

@@ -30,6 +30,24 @@
         .custom-drop-zone > *:not(#overviewDemoSelectButton) {
             pointer-events: none;
         }
+
+    .pdf-viewer {
+        height: 800px !important;
+        min-height: 800px !important;
+    }
+
+    .pdf-viewer .dxbrv-surface-wrapper,
+    .pdf-viewer .dxbrv-document-surface {
+        height: 100% !important;
+        min-height: 750px !important;
+    }
+
+    .pdf-viewer .dxbrv-report-preview-content {
+        width: auto !important;
+        height: auto !important;
+        min-width: 200px !important;
+        min-height: 200px !important;
+    }
 </style>
 
 <div id="overviewDemoDropZone" class="card custom-drop-zone rounded-3 w-100 m-0">
@@ -50,7 +68,20 @@
              FilesUploading="OnFilesUploading"
              MaxFileSize="2000000">
 </DxFileInput>
-<DxPdfViewer CssClass="w-100 pdf-viewer" DocumentContent="DocumentContent" />
+
+@if (DocumentContent != null && DocumentContent.Length > 0)
+{
+    <div class="alert alert-success mt-3">
+        PDF loaded: @DocumentContent.Length bytes
+    </div>
+    <DxPdfViewer CssClass="w-100 pdf-viewer" DocumentContent="@DocumentContent" />
+}
+else
+{
+    <div class="alert alert-info mt-3">
+        Please upload a PDF file to view it.
+    </div>
+}
 
 @code {
     readonly List<string> ALLOWED_FILE_TYPES = new List<string> { ".pdf" };
@@ -77,4 +108,5 @@
             await InvokeAsync(StateHasChanged);
         }
     }
-}
+}
+