Add unused vacation days field

[CesarOrdazV]

Adds an integer field for unused vacation days so they can be subtracted from expected work time.

Behavior

• New form field Vacaciones no disfrutadas (días).
• Integer only, defaults to 0, rejects negatives and decimals.
• Each unused vacation day subtracts 8 hours from expected hours (días laborables × 8 − vacaciones × 8).
• Working-day count and daily average still use the calendar working days.
• Existing calculations are unchanged when the field is 0.

Tests

node --test worktime.test.js covers default 0, subtraction of 8 h per day, and rejection of negatives/non-integers.