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/wwwroot/js/pdfInterop.js

@@ -175,7 +175,7 @@
                     x,
                     y: y - 14 * scaleY,
                     size: 14 * scaleX,
-                    color: PDFLib.rgb(0.07, 0.54, 0.26),
+                    color: PDFLib.rgb(0.11, 0.25, 0.56),
                 });
             }
 
@@ -209,7 +209,7 @@
                 x,
                 y,
                 size: fontSize * scaleX,
-                color: PDFLib.rgb(0.9, 0.9, 0.9),
+                color: PDFLib.rgb(0.2, 0.23, 0.28),
             });
 
             const updatedBase64 = await pdfDoc.saveAsBase64({ dataUri: false });
@@ -225,7 +225,7 @@
             const ctx = canvas.getContext('2d');
             ctx.lineWidth = 2;
             ctx.lineCap = 'round';
-            ctx.strokeStyle = '#22d3ee';
+            ctx.strokeStyle = '#1c3d8f';
 
             const padState = {
                 drawing: false,