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/app.js

@@ -1,6 +1,7 @@
 import { renderEmployees } from './employees.js';
 import { renderCalendar } from './calendar.js';
 import { renderDataTab } from './history.js';
+import { normalizeEmployees } from './data.js';
 
 const STORAGE_KEY = 'morning-planner-state';
 
@@ -17,7 +18,7 @@ try {
   const saved = localStorage.getItem(STORAGE_KEY);
   if (saved) {
     const parsed = JSON.parse(saved);
-    if (Array.isArray(parsed.employees)) state.employees = parsed.employees;
+    if (Array.isArray(parsed.employees)) state.employees = normalizeEmployees(parsed.employees);
     if (parsed.calendar) state.calendar = parsed.calendar;
     if (Array.isArray(parsed.history)) state.history = parsed.history;
     if (parsed.settings && Number.isInteger(parsed.settings.moderatorsPerDay)