* check if ffmpeg is faster with less input files
* close filehandle after sending?
  * or clean up when handling subsequent requests
* `DeprecationWarning: The 'attachment_filename' parameter has been renamed to 'download_name'. The old name will be removed in Flask 2.1.`

bootstrap 4
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* fix calendar icon
* fix calender locale ("December")
* waren die Fehler früher rot?

* add "Minuten" to Länge
* update screenshot