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/history.js

@@ -1,6 +1,7 @@
 import { state, saveState } from './app.js';
 import { loadFromJSON, saveToJSON, removeHistoryEntry } from './data.js';
 import { downloadBlob } from './excel.js';
+import { formatDateDE, buildNameMap } from './utils.js';
 
 export function renderDataTab() {
   const container = document.getElementById('tab-data');
@@ -102,7 +103,7 @@ function renderHistoryTable() {
     return;
   }
 
-  const nameById = Object.fromEntries(state.employees.map(e => [e.id, e.name]));
+  const nameById = buildNameMap(state.employees);
   const sorted = [...state.history].sort((a, b) => {
     const dateCmp = b.date.localeCompare(a.date);
     if (dateCmp !== 0) return dateCmp;
@@ -124,11 +125,10 @@ function renderHistoryTable() {
 
   const tbody = document.createElement('tbody');
   for (const h of sorted) {
-    const [y, m, d] = h.date.split('-');
     const tr = document.createElement('tr');
 
     const dateTd = document.createElement('td');
-    dateTd.textContent = d + '.' + m + '.' + y;
+    dateTd.textContent = formatDateDE(h.date);
 
     const nameTd = document.createElement('td');
     nameTd.textContent = nameById[h.employeeId] ?? '(unbekannt)'; // textContent: safe