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.Client/Pages/SenderEditorPage.razor

@@ -5,6 +5,7 @@
 @using EnvelopeGenerator.Application.Common.Dto
 @using EnvelopeGenerator.Application.Envelopes.Commands
 @using EnvelopeGenerator.Application.Resources
+@using EnvelopeGenerator.Server.Client.Components.PdfEditor
 @using EnvelopeGenerator.Server.Client.Services
 @using Microsoft.AspNetCore.WebUtilities
 @using Microsoft.AspNetCore.Components.Forms
@@ -336,6 +337,7 @@
                                   PdfDataUrl="@PdfDataUrl"
                                   Fields="@FieldOverlays"
                                   EditorClick="OnPdfAreaClickAsync"
+                                  MoveField="OnMoveFieldAsync"
                                   RemoveField="OnRemoveFieldByIndexAsync" />
         }
     </div>
@@ -806,6 +808,22 @@
         await Task.CompletedTask;
     }
 
+    async Task OnMoveFieldAsync(SenderPdfFieldMoveRequest request)
+    {
+        if (request.Index < 0 || request.Index >= _signatureFields.Count)
+            return;
+
+        var field = _signatureFields[request.Index];
+        _signatureFields[request.Index] = field with
+        {
+            XPt = request.XPt,
+            YPt = request.YPt,
+            Page = request.Page
+        };
+
+        await Task.CompletedTask;
+    }
+
     // ── Clear all fields ──
     async Task ClearAllFieldsAsync()
     {