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/calendar.js

@@ -28,8 +28,10 @@ export function renderCalendar() {
 
   const monthLabel = document.createElement('label');
   monthLabel.textContent = 'Monat:';
+  monthLabel.htmlFor = 'cal-month';
 
   const monthSel = document.createElement('select');
+  monthSel.id = 'cal-month';
   MONTHS.forEach((m, i) => {
     const opt = document.createElement('option');
     opt.value = String(i + 1);
@@ -40,8 +42,10 @@ export function renderCalendar() {
 
   const yearLabel = document.createElement('label');
   yearLabel.textContent = 'Jahr:';
+  yearLabel.htmlFor = 'cal-year';
 
   const yearSel = document.createElement('select');
+  yearSel.id = 'cal-year';
   const thisYear = new Date().getFullYear();
   for (let y = thisYear - 1; y <= thisYear + 2; y++) {
     const opt = document.createElement('option');
@@ -53,8 +57,10 @@ export function renderCalendar() {
 
   const slotsLabel = document.createElement('label');
   slotsLabel.textContent = 'Pro Tag:';
+  slotsLabel.htmlFor = 'cal-slots';
 
   const slotsSel = document.createElement('select');
+  slotsSel.id = 'cal-slots';
   for (let n = 1; n <= 5; n++) {
     const opt = document.createElement('option');
     opt.value = String(n);