New js/utils.js exports toISO, monthPrefix, getWeekday, formatDateDE and
buildNameMap. algorithm.js, calendar.js, excel.js and history.js now import
these instead of re-implementing ISO construction, weekday math, DD.MM.YYYY
formatting and the id→name map inline. Adds tests/utils.test.mjs.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>