fix(import): validate/normalize imported data, guard Excel I/O (#3)

- loadFromJSON and the localStorage startup load now run employees through
  normalizeEmployees(): fills missing constraints, drops unusable rows,
  de-duplicates ids, and discards invalid vacation ranges — preventing
  later crashes (e.g. c.neverDays on undefined) and same-day double-books.
- Excel import is wrapped in try/catch with a typeof XLSX guard and user
  feedback; the Excel export button gets the same guard.
- createEmployee reuses the shared defaultConstraints() factory.

Adds normalizeEmployees tests (defaults, dropping, dedup, vacations) and a
loadFromJSON normalization test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/calendar.js

@@ -138,6 +138,10 @@ export function renderCalendar() {
   });
 
   exportBtn.addEventListener('click', () => {
+    if (typeof XLSX === 'undefined') {
+      alert('Excel-Export nicht verfügbar (SheetJS konnte nicht geladen werden).');
+      return;
+    }
     const blob = exportSchedule(currentPlan, state.employees);
     downloadBlob(blob, 'Moderationsplan-' + MONTHS[currentMonth-1] + '-' + currentYear + '.xlsx');
   });