feat: add placeholder to card editor (#3870)

frontend/appflowy_flutter/lib/plugins/database_view/widgets/row/row_document.dart

@@ -116,6 +116,10 @@ class _RowEditorState extends State<RowEditor> {
                           context: context,
                           padding: const EdgeInsets.symmetric(horizontal: 10),
                         ),
+                        showParagraphPlaceholder: (editorState, node) =>
+                            editorState.document.isEmpty,
+                        placeholderText: (node) =>
+                            LocaleKeys.cardDetails_notesPlaceholder.tr(),
                       ),
                     ),
                   );