40a90c7ff5
- Add new_grid() method to create empty DataGrid under selected folder/leaf parent - Generate unique sheet names (Sheet1, Sheet2, ...) with _generate_unique_sheet_name() - Auto-select and open new node in edit mode for immediate renaming - Fix TreeView to cancel edit mode when selecting any node - Wire "New grid" icon to new_grid() instead of clear_tree() - Add 14 unit tests covering new_grid() scenarios and TreeView behavior