fix(dashboard): slide a drag back when a read-only folder refuses it

A drop a read-only shared folder refused left the drag chip to vanish
where it was let go, as if something had happened. It now slides back
to where the drag began, as any drop that doesn't land does.

src/gui/src/UI/Dashboard/TabFiles.js

@@ -3856,7 +3856,13 @@ const TabFiles = {
             // obscures the drop target under the cursor.
             cursorAt: { left: -14, top: -12 },
             cursor: 'grabbing',
-            revert: 'invalid',
+            // As 'invalid', and also when a folder refused the drop (a
+            // read-only share), so the chip slides back rather than vanishing.
+            revert: function (dropped) {
+                const refused = $(this).data('dropRefused');
+                $(this).removeData('dropRefused');
+                return ! dropped || !! refused;
+            },
             zIndex: 999999999,
             scroll: false,
             distance: 5,
@@ -4009,6 +4015,7 @@ const TabFiles = {
                     // would refuse each item. The drag goes back instead.
                     if ( _this.isReadOnlyShare($(el_item).attr('data-path')) ) {
                         ui.helper.data('cancelled', true);
+                        $(ui.draggable).data('dropRefused', true);
                         return;
                     }