Allow the file picker to select multiple epubs at once, and send them to the device sequentially in a loop, to make user flow easier for larger quantities of files. Currently you must wait for one file to upload, before re-opening the modal and selecting the next file, which you then have to wait for.

Also add some logic to retry files that fail (which seemed to happen a moderate amount in my testing, might just be a network problem on my end) without having to re-select and re-upload the files to the browser.

(PR incoming)