Enhance signature management functionality

Added a new button in `EnvelopeViewer.razor` for creating or modifying signatures, with dynamic styling and tooltips based on the signature state. Enhanced `OpenSignaturePopup` and `OnPopupShownAsync` methods to preload and display existing signatures in the popup and canvas.

Introduced new "success" button styles in `envelope-viewer.css` for better visual feedback. Added `loadExistingSignature` function in `receiver-signature.js` to render existing signatures on the canvas and updated the public API to expose this functionality.

EnvelopeGenerator.ReceiverUI/wwwroot/js/receiver-signature.js

@@ -316,6 +316,23 @@ window.receiverSignature = (() => {
     function getTypedDataUrl(id) { const c = document.getElementById(id); const s = typedSignatures.get(id); return (c && s && s.hasSignature) ? c.toDataURL('image/png') : null; }
     function getImageDataUrl(id) { const c = document.getElementById(id); const s = imageSignatures.get(id); return (c && s && s.hasSignature) ? c.toDataURL('image/png') : null; }
 
+    function loadExistingSignature(canvasId, dataUrl) {
+        const canvas = document.getElementById(canvasId);
+        const state = pads.get(canvasId);
+        if (!canvas || !state || !dataUrl) return;
+
+        const ctx = canvas.getContext('2d');
+        const img = new Image();
+        
+        img.onload = () => {
+            _clear(canvas);
+            ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
+            state.hasSignature = true;
+        };
+        
+        img.src = dataUrl;
+    }
+
     // ?? Public API ??????????????????????????????????????????????????????????
     return {
         startTyped:                  startTyped,
@@ -329,6 +346,8 @@ window.receiverSignature = (() => {
         renderTypedSignature:        renderTypedSignature,
         getDataUrl:                  getDataUrl,
         getTypedDataUrl:             getTypedDataUrl,
-        getImageDataUrl:             getImageDataUrl
+        getImageDataUrl:             getImageDataUrl,
+        loadExistingSignature:       loadExistingSignature
     };
 })();
+