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>

js/data.js

@@ -109,6 +109,8 @@ export function createEmployee(name) {
 
 export function removeEmployee(state, id) {
   state.employees = state.employees.filter(e => e.id !== id);
+  // Also drop the employee's history entries so no orphaned "(unbekannt)" rows remain
+  state.history = state.history.filter(h => h.employeeId !== id);
 }
 
 export function toggleHoliday(state, isoDate) {