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/utils.js

@@ -0,0 +1,28 @@
+// Shared pure helpers used across modules.
+
+// "YYYY-MM-DD" from numeric parts.
+export function toISO(year, month, day) {
+  return year + '-' + String(month).padStart(2, '0') + '-' + String(day).padStart(2, '0');
+}
+
+// "YYYY-MM" month prefix (used to filter history by month).
+export function monthPrefix(year, month) {
+  return year + '-' + String(month).padStart(2, '0');
+}
+
+// Weekday for an ISO date string: 1=Mo, 2=Di, 3=Mi, 4=Do, 5=Fr, 6=Sa, 7=So.
+export function getWeekday(isoDate) {
+  const d = new Date(isoDate + 'T12:00:00Z').getUTCDay(); // 0=Sun
+  return d === 0 ? 7 : d;
+}
+
+// "DD.MM.YYYY" from an ISO date string.
+export function formatDateDE(isoDate) {
+  const [y, m, d] = isoDate.split('-');
+  return d + '.' + m + '.' + y;
+}
+
+// { [id]: name } lookup map from an employees array.
+export function buildNameMap(employees) {
+  return Object.fromEntries(employees.map(e => [e.id, e.name]));
+}