[29202] Don't open editor unless readable on drop attachment

https://community.openproject.com/wp/29202

frontend/src/app/components/wp-edit/wp-edit-field/wp-edit-field.component.ts

@@ -93,7 +93,7 @@ export class WorkPackageEditFieldComponent implements OnInit {
 
   // Open the field when its closed and relay drag & drop events to it.
   public startDragOverActivation(event:JQueryEventObject) {
-    if (!this.isDropTarget || this.active) {
+    if (!this.isDropTarget || !this.isEditable || this.active) {
       return true;
     }