Resume or retry stopped, interrupted, or failed uploads

if i'm uploading a large number of files, and i accidentally click away from the upload window or lose network connectivity, the upload's individual progress bars will just appear to freeze and not move, given the impression that it might still be uploading.

Here's what i think could be done to make the upload flow more clear and intuitive:

- add a button to explicitly cancel uploads
- add a confirmation when the user clicks outside of the upload popup (something like "uploads are still in progress, do you really want to cancel them?")
- detect when network connectivity is lost (iirc there is an `online` and `offline` event) and pop up an alert informing the user that they are offline and asking if they want to retry the uploads that failed
- if it's just a single upload that failed, let the user know (maybe show a semitransparent refresh icon over it to indicate that the user should click it to retry) and/or automatically retry until a retry limit is hit

Comienza con la ventana emergente de carga y sus barras de progreso individuales, y después revisa cómo se relacionan los eventos online/offline del navegador con las cargas fallidas. Se considera terminado cuando los usuarios pueden cancelar cargas deliberadamente, reciben una advertencia antes de salir mientras hay cargas activas y pueden identificar y reintentar las cargas fallidas después de perder la conectividad.

Escrito por el modelo de indexación a partir del texto del issue.