refactor: centralize shared date/name helpers in utils.js (#12)

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>

js/algorithm.js

@@ -1,13 +1,4 @@
-function toISO(year, month, day) {
-  return year + '-' + String(month).padStart(2, '0') + '-' + String(day).padStart(2, '0');
-}
-
-// Returns 1=Mo, 2=Di, 3=Mi, 4=Do, 5=Fr, 6=Sa, 7=So
-function getWeekday(isoDate) {
-  const d = new Date(isoDate + 'T12:00:00Z');
-  const day = d.getUTCDay(); // 0=Sun
-  return day === 0 ? 7 : day;
-}
+import { toISO, getWeekday } from './utils.js';
 
 function isOnVacation(employee, isoDate) {
   return employee.constraints.vacations.some(v => v.from <= isoDate && isoDate <= v.to);