Enhance Excel export functionality in backend and frontend; implement Excel file generation and update download logic for correct file extension

frontend/src/views/Export.vue

@@ -153,7 +153,11 @@ async function downloadFile(format) {
   const downloadUrl = window.URL.createObjectURL(blob)
   const a = document.createElement('a')
   a.href = downloadUrl
-  a.download = `zeiterfassung_${form.value.startDate}_${form.value.endDate}.csv`
+  
+  // Richtiger Dateiname je nach Format
+  const extension = format === 'excel' ? 'xlsx' : 'csv'
+  a.download = `zeiterfassung_${form.value.startDate}_${form.value.endDate}.${extension}`
+  
   document.body.appendChild(a)
   a.click()
   document.body.removeChild(a)