a11y: associate form labels with inputs (#15)

Month/year/slots selects get ids with matching <label for>; the employee name
input, the Excel import file input and the JSON import file input get
aria-labels so screen readers announce them.

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

js/employees.js

@@ -22,6 +22,7 @@ export function renderEmployees() {
   nameInput.id = 'emp-name-input';
   nameInput.type = 'text';
   nameInput.placeholder = 'Name eingeben...';
+  nameInput.setAttribute('aria-label', 'Mitarbeitername');
   nameInput.style.width = '220px';
 
   const addBtn = document.createElement('button');
@@ -37,6 +38,7 @@ export function renderEmployees() {
   fileInput.id = 'emp-excel-input';
   fileInput.type = 'file';
   fileInput.accept = '.xlsx,.xls';
+  fileInput.setAttribute('aria-label', 'Excel-Datei mit Mitarbeiternamen importieren');
   fileInput.style.display = 'none';
 
   row.append(nameInput, addBtn, excelBtn, fileInput);