Add drag-and-drop support for PDF field repositioning

Introduced drag-and-drop functionality for fields in the PDF editor:
- Added `SenderPdfFieldMoveRequest` record to encapsulate field movement data.
- Enhanced `SenderPdfImageEditor.razor` with drag-and-drop event handlers (`OnFieldDragStart`, `OnPageDropAsync`) and updated field structure.
- Updated `SenderEditorPage.razor` to handle field movement via the `OnMoveFieldAsync` method.
- Improved CSS styles for draggable fields and added a delete button with hover effects.
- Added `getDropCoordsOnPage` function in `envelope-editor.js` to calculate normalized drop coordinates.

These changes improve the user experience by enabling intuitive field placement and management in the PDF editor.

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

@@ -134,13 +134,47 @@
     padding: 0.2rem 0.3rem;
     text-align: left;
     line-height: 1.1;
-    cursor: pointer;
+    cursor: grab;
     display: flex;
     align-items: flex-start;
-    justify-content: flex-start;
+    justify-content: space-between;
+    gap: 0.35rem;
     overflow: hidden;
 }
 
+.sender-image-editor__field:active {
+    cursor: grabbing;
+}
+
+.sender-image-editor__field-label {
+    min-width: 0;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
+.sender-image-editor__field-remove {
+    border: none;
+    background: rgba(15, 23, 42, 0.78);
+    color: #ffffff;
+    width: 1.1rem;
+    height: 1.1rem;
+    min-width: 1.1rem;
+    border-radius: 999px;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    padding: 0;
+    cursor: pointer;
+    line-height: 1;
+    margin-left: auto;
+    flex-shrink: 0;
+}
+
+.sender-image-editor__field-remove:hover {
+    background: rgba(220, 38, 38, 0.9);
+}
+
 .sender-image-editor__field:hover {
     filter: brightness(0.96);
 }

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

@@ -92,5 +92,31 @@ window.envelopeEditor = {
             normY:     normY,
             pageIndex: targetIndex
         };
+    },
+
+    getDropCoordsOnPage: function (viewerCssClass, pageNumber, clientX, clientY) {
+        const viewer = document.querySelector('.' + viewerCssClass);
+        if (!viewer) {
+            console.warn('[envelopeEditor] viewer not found for class:', viewerCssClass);
+            return null;
+        }
+
+        const page = viewer.querySelector('[data-editor-page="' + pageNumber + '"]');
+        if (!page) {
+            console.warn('[envelopeEditor] page not found:', pageNumber);
+            return null;
+        }
+
+        const rect = page.getBoundingClientRect();
+        const clampedX = Math.max(rect.left, Math.min(clientX, rect.right));
+        const clampedY = Math.max(rect.top, Math.min(clientY, rect.bottom));
+
+        const normX = (clampedX - rect.left) / rect.width;
+        const normY = (clampedY - rect.top) / rect.height;
+
+        return {
+            normX: normX,
+            normY: normY
+        };
     }
 };