Refactor PDF editor with custom rendering

Replaced `DxPdfViewer` with a custom PDF rendering solution in `SenderPdfImageEditor` to improve flexibility and control. Added support for displaying placeholder messages and interactive signature fields. Introduced the `SenderPdfFieldOverlay` record to encapsulate field data.

Updated `SenderEditorPage` logic to manage page counts, image URLs, and field overlays. Added a method to estimate PDF page counts. Refactored field removal logic to use index-based operations.

Enhanced styling in `envelope-viewer.css` for improved layout and responsiveness. Updated localization files with new strings for page labels and placeholders. Adjusted JavaScript selectors in `envelope-editor.js` to support the new rendering approach.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/envelope-viewer.css

@@ -56,6 +56,104 @@
     height: 100%;
 }
 
+.sender-image-editor {
+    overflow: auto;
+    padding: 1rem;
+}
+
+.sender-image-editor__pages {
+    max-width: 980px;
+    margin: 0 auto;
+    display: flex;
+    flex-direction: column;
+    gap: 1.25rem;
+}
+
+.sender-image-editor__native-preview {
+    max-width: 980px;
+    margin: 0 auto 1rem;
+    background: #ffffff;
+    border: 1px solid #cbd5e1;
+    border-radius: 10px;
+    overflow: hidden;
+    box-shadow: 0 8px 20px rgba(15, 23, 42, 0.12);
+}
+
+.sender-image-editor__native-frame {
+    width: 100%;
+    min-height: 70vh;
+    border: 0;
+    background: #f8fafc;
+}
+
+.sender-image-editor__page-block {
+    background: #e2e8f0;
+    border: 1px solid #cbd5e1;
+    border-radius: 10px;
+    padding: 0.75rem;
+}
+
+.sender-image-editor__page-canvas {
+    position: relative;
+    width: 100%;
+    aspect-ratio: 595 / 842;
+    background: #ffffff;
+    border-radius: 8px;
+    overflow: hidden;
+    box-shadow: 0 8px 20px rgba(15, 23, 42, 0.12);
+}
+
+.sender-image-editor__page-image {
+    width: 100%;
+    height: 100%;
+    object-fit: contain;
+    display: block;
+    pointer-events: none;
+}
+
+.sender-image-editor__page-placeholder {
+    position: absolute;
+    inset: 0;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    color: #64748b;
+    font-weight: 600;
+    font-size: 0.95rem;
+    letter-spacing: 0.01em;
+    background: linear-gradient(135deg, #f8fafc 0%, #f1f5f9 100%);
+}
+
+.sender-image-editor__field {
+    position: absolute;
+    border: 2px solid;
+    border-radius: 8px;
+    font-size: 0.72rem;
+    font-weight: 700;
+    color: #0f172a;
+    padding: 0.2rem 0.3rem;
+    text-align: left;
+    line-height: 1.1;
+    cursor: pointer;
+    display: flex;
+    align-items: flex-start;
+    justify-content: flex-start;
+    overflow: hidden;
+}
+
+.sender-image-editor__field:hover {
+    filter: brightness(0.96);
+}
+
+.sender-image-editor__empty {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    min-height: 320px;
+    color: #64748b;
+    font-weight: 600;
+}
+
 .sender-editor-pdf-viewer {
     width: 100%;
     height: 100%;

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

@@ -25,9 +25,10 @@ window.envelopeEditor = {
         }
 
         // --- Candidate page elements (ordered by preference) ---
-        // DxPdfViewer renders individual pages as .dxbl-pdfv-page elements.
-        // DxReportViewer uses .dxbrv-report-preview-content-img as fallback.
+        // Sender editor custom renderer uses data-editor-page blocks.
+        // Legacy Dx viewers remain as fallback selectors.
         const pageSelectors = [
+            '[data-editor-page]',
             '.dxbl-pdfv-page',
             '.dxbrv-report-preview-page',
             '.dxbrv-report-preview-content-img',