fix: add days (d) unit support to parse_duration

[qyu450724-source]

Summary

The parse_duration function was missing support for the days (d) unit, despite the regex pattern already matching it in [wdhms].

Changes

• Added "d": 86400 to the _UNITS dictionary so that parse_duration("1d") correctly returns 86400
• Updated the comment to reflect the new supported units list
• Added 4 new test cases:
  • test_days: 1d -> 86400
  • test_days_multiple: 3d -> 259200
  • test_days_combined_with_hours: 2d4h -> 187200
  • test_full_combined: 1w2d3h4m5s -> 788645

All 11 tests pass.

Fixes #1