Improve PDF viewer layout and styling

Updated `.pdf-viewer-container` to vertically center content by changing `align-items` to `center`. Adjusted `.pdf-frame` dimensions to use dynamic height and width, added a maximum width, and centered text alignment. Introduced rounded corners to the gradient bar with a `border-radius`. Changed `.pdf-canvas` to `inline-block` display with `vertical-align: top`. Further refined `.pdf-frame` layout with slightly different height and width calculations.

EnvelopeGenerator.ReceiverUI/wwwroot/css/envelope-viewer.css

@@ -68,7 +68,7 @@
 .pdf-viewer-container {
     height: 100%;
     display: flex;
-    align-items: flex-start;
+    align-items: center;
     justify-content: center;
     padding: 0;
 }
@@ -82,8 +82,10 @@
     overflow: auto;
     position: relative;
     padding: 2rem;
-    max-height: 100%;
-    width: auto;
+    height: calc(100vh - 200px);
+    width: 90%;
+    max-width: 1200px;
+    text-align: center;
 }
 
 .pdf-frame::before {
@@ -95,12 +97,13 @@
     height: 4px;
     background: linear-gradient(90deg, #7e22ce 0%, #2a5298 100%);
     z-index: 1;
+    border-radius: 16px 16px 0 0;
 }
 
 .pdf-canvas {
-    display: block;
+    display: inline-block;
     box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
-    margin: 0 auto;
+    vertical-align: top;
 }
 
 .error-container {
@@ -142,6 +145,8 @@
     .pdf-frame {
         border-radius: 12px;
         padding: 1rem;
+        height: calc(100vh - 180px);
+        width: 95%;
     }
 
     .envelope-action-bar {