Set busy cursor while loading environment

[michaellans]

Introduces a new gui util for setting the cursor to provide visual feedback during slow GUI operations.

• Added with_busy_cursor decorator to gui/utils.py, which wraps the decorated function with set_busy_cursor and unset_busy_cursor. The cursor indicates a busy state while a decorated function runs, and resets afterward.

• Applied @with_busy_cursor to select_env in routine_page.py

• Added unset_busy_cursor in expandable_message_box.py so that if an error message is shown, the cursor will revert to normal.