Add configurable PDF viewer options and improve rendering

Introduced `PdfViewerOptions` class to centralize PDF viewer
settings such as scaling, HiDPI support, zoom transitions,
and rendering delays. Bound these options to `appsettings.json`
for dynamic configuration.

Injected `PdfViewerOptions` into `EnvelopeViewer.razor` and
updated `OnInitializedAsync` to pass settings to JavaScript.
Replaced hardcoded values in `pdf-viewer.js` with configurable
options, improving maintainability and flexibility.

Enhanced rendering logic to respect HiDPI settings, maximum
DPR, and smooth zoom transitions. Improved thumbnail rendering
with configurable delays to optimize performance.

EnvelopeGenerator.ReceiverUI/wwwroot/appsettings.json

@@ -2,5 +2,16 @@
   "Api": {
     "BaseUrl": "",
     "ForceToUseFakeDocument": false
+  },
+  "PdfViewer": {
+    "ThumbnailBaseScale": 0.75,
+    "ThumbnailEnableHiDPI": true,
+    "ThumbnailMaxDPR": 2.0,
+    "MainCanvasEnableHiDPI": true,
+    "MainCanvasMaxDPR": 2.0,
+    "EnableSmoothZoom": true,
+    "ZoomTransitionDuration": 900,
+    "RenderingOpacity": 0.85,
+    "ThumbnailRenderDelay": 50
   }
 }