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/history.js

@@ -40,6 +40,7 @@ export function renderDataTab() {
   const fileInput = document.createElement('input');
   fileInput.type = 'file';
   fileInput.accept = '.json';
+  fileInput.setAttribute('aria-label', 'JSON-Datensicherung importieren');
   fileInput.style.display = 'none';
   loadRow.append(loadBtn, fileInput);
   saveLoadCol.appendChild(loadRow);