Function data_dir 

pub fn data_dir() -> Result<PathBuf>

Base data directory for Task Journal on the current OS.

Resolution order (first wins):

1. TASK_JOURNAL_DATA_DIR env (explicit override; portable across all OS)
2. XDG_DATA_HOME env (Linux/WSL convention; respected on every OS for testability)
3. OS default via directories crate:
   • Linux/WSL: ~/.local/share/task-journal
   • macOS: ~/Library/Application Support/task-journal
   • Windows: %LOCALAPPDATA%\task-journal