fix(employees): remove history entries when deleting an employee (#5)

removeEmployee now also filters the deleted employee's history entries, so
no orphaned "(unbekannt)" rows remain. Adds a test.

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

tests/data.test.mjs

@@ -1,5 +1,5 @@
 import assert from 'node:assert/strict';
-import { addHistoryEntries, removeHistoryEntry, loadFromJSON, saveToJSON, normalizeEmployees } from '../js/data.js';
+import { addHistoryEntries, removeHistoryEntry, loadFromJSON, saveToJSON, normalizeEmployees, removeEmployee } from '../js/data.js';
 
 function makeState(history = []) {
   return {
@@ -164,4 +164,20 @@ function makeState(history = []) {
   assert.ok(Array.isArray(loaded.employees[0].constraints.neverDays), 'constraints present after load');
 }
 
+// removeEmployee: also removes the employee's history entries (no orphans)
+{
+  const state = {
+    employees: [{ id: 'a', name: 'Alice' }, { id: 'b', name: 'Bob' }],
+    calendar: { holidays: [], companyClosures: [] },
+    history: [
+      { date: '2026-04-01', employeeId: 'a', manual: false },
+      { date: '2026-04-02', employeeId: 'b', manual: false }
+    ]
+  };
+  removeEmployee(state, 'a');
+  assert.equal(state.employees.length, 1, 'alice removed from employees');
+  assert.ok(!state.history.some(h => h.employeeId === 'a'), 'alice history removed');
+  assert.equal(state.history.length, 1, 'bob history untouched');
+}
+
 console.log('All data tests passed.');