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/Pages/EnvelopeViewer.razor

@@ -6,6 +6,7 @@
 @inject DocumentService DocumentService
 @inject NavigationManager Navigation
 @inject IOptions<ApiOptions> AppOptions
+@inject IOptions<PdfViewerOptions> PdfViewerOptions
 @inject IJSRuntime JSRuntime
 @inject AnnotationService AnnotService
 @implements IAsyncDisposable
@@ -236,6 +237,21 @@ protected override async Task OnInitializedAsync() {
             
             try {
                 _dotNetRef = DotNetObjectReference.Create(this);
+                
+                // Send quality options to JavaScript
+                var options = PdfViewerOptions.Value;
+                await JSRuntime.InvokeVoidAsync("pdfViewer.setQualityOptions", new
+                {
+                    options.ThumbnailBaseScale,
+                    options.ThumbnailEnableHiDPI,
+                    options.ThumbnailMaxDPR,
+                    options.MainCanvasEnableHiDPI,
+                    options.MainCanvasMaxDPR,
+                    options.EnableSmoothZoom,
+                    options.ZoomTransitionDuration,
+                    options.RenderingOpacity
+                });
+                
                 var success = await JSRuntime.InvokeAsync<bool>("pdfViewer.initialize", "pdf-canvas", _pdfDataUrl, _dotNetRef);
                 
                 if (success) {
@@ -337,13 +353,15 @@ protected override async Task OnInitializedAsync() {
 
     async Task RenderThumbnailsAsync() {
         try {
+            var delay = PdfViewerOptions.Value.ThumbnailRenderDelay;
+            
             // Sequential rendering to avoid overwhelming the browser
             for (int i = 1; i <= _totalPages; i++) {
                 await JSRuntime.InvokeVoidAsync("pdfViewer.renderThumbnail", i, $"thumb-canvas-{i}");
                 
-                // Small delay between renders to keep UI responsive
+                // Configurable delay between renders
                 if (i < _totalPages) {
-                    await Task.Delay(50);
+                    await Task.Delay(delay);
                 }
             }
         } catch (Exception ex) {