Add PDF download functionality to editor components

Introduced the ability to download PDFs in the `SenderPdfImageEditor` component:
- Added `PdfFileName` parameter to specify the file name.
- Implemented `CanDownloadPdf` property to enable/disable downloads.
- Created `OnDownloadPdfAsync` method to handle download logic via JS interop.

Updated `SenderPdfImageEditorToolbar` to support downloads:
- Added `CanDownloadPdf` and `DownloadPdf` parameters.
- Enabled "Download PDF" button based on `CanDownloadPdf`.
- Added `DownloadPdfAsync` method to trigger the download callback.

Enhanced `SenderEditorPage` to pass `PdfFileName` to `SenderPdfImageEditor`.

Added `downloadDataUrlAsFile` function in `envelope-editor.js` to facilitate file downloads using a temporary `<a>` element.

Made minor adjustments for consistency, including adding `ZoomStepPercent` to the toolbar and refining `SetZoomPercent` logic.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/js/envelope-editor.js

@@ -225,5 +225,23 @@ window.envelopeEditor = {
 
         viewer.addEventListener('touchmove', function () {
         }, { passive: true });
+    },
+
+    downloadDataUrlAsFile: function (dataUrl, fileName, mimeType) {
+        if (!dataUrl || typeof dataUrl !== 'string') {
+            return;
+        }
+
+        const downloadName = fileName || 'document.pdf';
+
+        const link = document.createElement('a');
+        link.href = dataUrl;
+        link.download = downloadName;
+        link.rel = 'noopener';
+
+        const host = document.body || document.documentElement;
+        host.appendChild(link);
+        link.click();
+        host.removeChild(link);
     }
 };