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/calendar.js

@@ -2,6 +2,7 @@ import { state, saveState } from './app.js';
 import { getWorkdays, generatePlan } from './algorithm.js';
 import { toggleHoliday, toggleClosure, addHistoryEntries, removeHistoryEntry } from './data.js';
 import { exportSchedule, downloadBlob } from './excel.js';
+import { toISO, monthPrefix, getWeekday, buildNameMap } from './utils.js';
 
 const MONTHS = ['Januar','Februar','März','April','Mai','Juni',
   'Juli','August','September','Oktober','November','Dezember'];
@@ -122,7 +123,7 @@ export function renderCalendar() {
     }
     const workdays = getWorkdays(currentYear, currentMonth,
       state.calendar.holidays, state.calendar.companyClosures);
-    const prefix = currentYear + '-' + String(currentMonth).padStart(2, '0');
+    const prefix = monthPrefix(currentYear, currentMonth);
     // Preserve manual picks for this month; only auto-entries get regenerated.
     const manualEntries = state.history.filter(h => h.date.startsWith(prefix) && h.manual);
     const historyWithoutMonth = state.history.filter(h => !h.date.startsWith(prefix));
@@ -148,7 +149,7 @@ export function renderCalendar() {
 }
 
 function loadPlanFromHistory() {
-  const prefix = currentYear + '-' + String(currentMonth).padStart(2,'0');
+  const prefix = monthPrefix(currentYear, currentMonth);
   currentPlan = state.history
     .filter(h => h.date.startsWith(prefix))
     .map(h => ({ ...h }));
@@ -158,7 +159,7 @@ function renderGrid(wrapper, exportBtn) {
   wrapper.replaceChildren();
 
   const daysInMonth = new Date(currentYear, currentMonth, 0).getDate();
-  const empNameById = Object.fromEntries(state.employees.map(e => [e.id, e.name]));
+  const empNameById = buildNameMap(state.employees);
   const updateExportVisibility = () => {
     exportBtn.style.display = currentPlan.length > 0 ? 'inline-block' : 'none';
   };
@@ -166,8 +167,8 @@ function renderGrid(wrapper, exportBtn) {
   // Build one day cell. Self-contained so a change to a single day re-renders
   // just that cell (cell.replaceWith) instead of rebuilding the whole grid.
   function makeDayCell(d) {
-    const iso = currentYear + '-' + String(currentMonth).padStart(2,'0') + '-' + String(d).padStart(2,'0');
-    const wdNum = (() => { const x = new Date(iso + 'T12:00:00Z').getUTCDay(); return x === 0 ? 7 : x; })();
+    const iso = toISO(currentYear, currentMonth, d);
+    const wdNum = getWeekday(iso);
     const isWeekend = wdNum >= 6;
     const isHoliday = state.calendar.holidays.includes(iso);
     const isClosure = state.calendar.companyClosures.includes(iso);
@@ -291,8 +292,8 @@ function renderGrid(wrapper, exportBtn) {
   bodyGrid.className = 'calendar-body-grid';
 
   // Offset empty cells before first day
-  const firstISO = currentYear + '-' + String(currentMonth).padStart(2,'0') + '-01';
-  const firstWD = (() => { const d = new Date(firstISO + 'T12:00:00Z').getUTCDay(); return d === 0 ? 7 : d; })();
+  const firstISO = toISO(currentYear, currentMonth, 1);
+  const firstWD = getWeekday(firstISO);
   for (let i = 1; i < firstWD; i++) {
     const empty = document.createElement('div');
     empty.className = 'cal-day empty-cell';