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/data.js

@@ -19,6 +19,52 @@ function parseSettings(raw) {
   return { moderatorsPerDay: valid ? n : 1 };
 }
 
+function defaultConstraints() {
+  return {
+    neverDays: [],
+    lowPriorityDays: [],
+    onlyDays: [],
+    maxPerMonth: null,
+    minGapDays: 14,
+    vacations: []
+  };
+}
+
+// Coerce an arbitrary constraints object into a valid, fully-populated one.
+function normalizeConstraints(c) {
+  const raw = (c && typeof c === 'object') ? c : {};
+  const arr = v => (Array.isArray(v) ? v : []);
+  return {
+    neverDays: arr(raw.neverDays),
+    lowPriorityDays: arr(raw.lowPriorityDays),
+    onlyDays: arr(raw.onlyDays),
+    maxPerMonth: Number.isFinite(raw.maxPerMonth) ? raw.maxPerMonth : null,
+    minGapDays: Number.isFinite(raw.minGapDays) ? raw.minGapDays : 14,
+    vacations: arr(raw.vacations).filter(v =>
+      v && typeof v.from === 'string' && typeof v.to === 'string' && v.from <= v.to
+    )
+  };
+}
+
+// Validate/repair an imported employee list: drop unusable rows, fill missing
+// constraints, and deduplicate ids so downstream code never crashes or
+// double-books. Returns a fresh, safe array.
+export function normalizeEmployees(rawList) {
+  if (!Array.isArray(rawList)) return [];
+  const seen = new Set();
+  const out = [];
+  for (const e of rawList) {
+    if (!e || typeof e !== 'object') continue;
+    const name = String(e.name ?? '').trim();
+    if (!name) continue; // unusable without a name
+    let id = (typeof e.id === 'string' && e.id) ? e.id : generateId();
+    if (seen.has(id)) id = generateId(); // resolve duplicate ids
+    seen.add(id);
+    out.push({ id, name, constraints: normalizeConstraints(e.constraints) });
+  }
+  return out;
+}
+
 export function loadFromJSON(json) {
   let parsed;
   try {
@@ -30,7 +76,7 @@ export function loadFromJSON(json) {
     throw new Error('Ungültiges Dateiformat: Objekt erwartet');
   }
   return {
-    employees: Array.isArray(parsed.employees) ? parsed.employees : [],
+    employees: normalizeEmployees(parsed.employees),
     calendar: (parsed.calendar && typeof parsed.calendar === 'object')
       ? {
           holidays: Array.isArray(parsed.calendar.holidays) ? parsed.calendar.holidays : [],
@@ -57,14 +103,7 @@ export function createEmployee(name) {
   return {
     id: generateId(),
     name: trimmed,
-    constraints: {
-      neverDays: [],
-      lowPriorityDays: [],
-      onlyDays: [],
-      maxPerMonth: null,
-      minGapDays: 14,
-      vacations: []
-    }
+    constraints: defaultConstraints()
   };
 }