fix(chat): prevent sortable sidebar drags from triggering file upload overlay (#25675)

Add a custom MIME type (application/x-open-webui-drag) to intentional
chat and folder drag sources. The onDragOver handler in MessageInput
now checks for this type instead of the generic text/plain, which
SortableJS also sets during reorder operations for pinned menu items
(Notes, Workspace) and pinned Models.

src/lib/components/common/Folder.svelte

@@ -88,6 +88,10 @@
 						} finally {
 							draggedOver = false;
 						}
+
+						// Only process the first non-file item; all share the same
+						// text/plain payload, so continuing would duplicate the drop.
+						break;
 					}
 				}
 			}