Improve PDF reset logic and modernize UI styling

Refactor PDF reset to restore original document state using a new OriginalPdfBase64 variable and async logic. Redesign app.css with a lighter color palette, CSS variables, and updated styles for buttons, overlays, modals, and inputs for a cleaner, more accessible UI. Adjust signature and text overlay colors in pdfInterop.js for better contrast and consistency.

EnvelopeGenerator.ReceiverUIBlazor/Pages/Index.razor

@@ -84,6 +84,7 @@
     private ElementReference OverlayRef;
 
     private string? PdfBase64;
+    private string? OriginalPdfBase64;
     private int PageIndex;
     private int PageCount;
     private double ViewportWidthPx = 800;
@@ -143,6 +144,7 @@
             using var ms = new MemoryStream();
             await stream.CopyToAsync(ms);
             PdfBase64 = Convert.ToBase64String(ms.ToArray());
+            OriginalPdfBase64 = PdfBase64;
 
             // Show the canvas before we start rendering
             await InvokeAsync(StateHasChanged);
@@ -186,13 +188,28 @@
         }
     }
 
-    private void Reset()
+    private async Task Reset()
     {
-        PdfBase64 = null;
-        PageIndex = 0;
-        PageCount = 0;
-        ViewportHeightPx = 0;
+        ErrorMessage = null;
         CloseOverlays();
+        ShowSignaturePadModal = false;
+        OverlayXpx = 20;
+        OverlayYpx = 20;
+        OverlayWidthPx = 200;
+        OverlayHeightPx = 80;
+        TextValue = "Text";
+
+        if (string.IsNullOrWhiteSpace(OriginalPdfBase64))
+        {
+            return;
+        }
+
+        PdfBase64 = OriginalPdfBase64;
+        PageIndex = 0;
+
+        var result = await JS.InvokeAsync<RenderResult>("pdfInterop.loadPdf", PdfBase64);
+        PageCount = result.Pages;
+        await RenderPage();
     }
 
     private void CloseOverlays()