- Add _parse_natural() to utils.py using dateparser as a fallback when
structured date formats (YYYY-MM-DD, MM-DD, DD-MM) don't match
- Supports expressions like 'today', 'yesterday', 'monday', '3 days ago'
- Change day argument to nargs='*' and join tokens so unquoted
multi-word expressions like: uv run timesheets 3 days ago work correctly
- Pin dateparser to English to avoid locale-dependent behaviour
- Update tests to cover natural language cases and fix test_last_monday
(dateparser does not support 'last monday'; use 'monday' instead)
- Add parse_date_arg() to utils.py supporting YYYY-MM-DD, MM-DD, and
DD-MM formats with either - or / as separator
- Add AmbiguousDateError for two-part dates valid as both MM-DD and DD-MM
- Replace --day flag with a positional optional argument (defaults to today)
- Remove old _parse_date() helper from cli.py
