- 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