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/employees.js

@@ -69,16 +69,31 @@ export function renderEmployees() {
   fileInput.addEventListener('change', async e => {
     const file = e.target.files[0];
     if (!file) return;
-    const buf = await file.arrayBuffer();
-    const names = readEmployeeNames(buf);
-    for (const name of names) {
-      if (!state.employees.some(emp => emp.name === name)) {
-        try { state.employees.push(createEmployee(name)); } catch { /* skip invalid */ }
+    if (typeof XLSX === 'undefined') {
+      alert('Excel-Funktion nicht verfügbar (SheetJS konnte nicht geladen werden).');
+      e.target.value = '';
+      return;
+    }
+    try {
+      const buf = await file.arrayBuffer();
+      const names = readEmployeeNames(buf);
+      if (names.length === 0) {
+        alert('Keine Namen in der Datei gefunden.');
+      } else {
+        let added = 0;
+        for (const name of names) {
+          if (!state.employees.some(emp => emp.name === name)) {
+            try { state.employees.push(createEmployee(name)); added++; } catch { /* skip invalid */ }
+          }
+        }
+        saveState();
+        refreshList();
+        if (added === 0) alert('Alle Namen waren bereits vorhanden.');
       }
+    } catch (err) {
+      alert('Excel-Import fehlgeschlagen: ' + (err?.message || err));
     }
     e.target.value = '';
-    saveState();
-    refreshList();
   });
 }