Fix Excel format detection for old binary .xls files (v0.8.1) (#16)

* Fix Excel format detection for old binary .xls files (v0.8.1)

Add automatic detection of Excel file format (binary .xls vs .xlsx) by
reading magic bytes. This fixes import errors when using old binary .xls
credit card statements.

- Add _detect_excel_engine() method to auto-detect xlrd vs openpyxl
- Add xlwt to requirements for binary .xls write support
- Add test fixture with old binary .xls format
- Add test for binary format extraction

* Fix line length in test (ruff check)

* Run ruff format to fix formatting