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

Beginne mit dem Upload-Popup und seinen einzelnen Fortschrittsbalken und überprüfe anschließend, wie die Online-/Offline-Ereignisse des Browsers mit fehlgeschlagenen Uploads zusammenhängen. Fertig ist die Aufgabe, wenn Benutzer Uploads bewusst abbrechen können, vor dem Verlassen aktiver Uploads eine Warnung erhalten und fehlgeschlagene Uploads nach einem Verbindungsverlust erkennen und erneut versuchen können.

Vom Indexierungsmodell aus dem Issue-Text verfasst.