Add resume

[Tragen]

It would be so great to have the option to resume files.
Also if I transfer a folder and I receive 10 out of 50 files and the sender aborts the transfer, the files are gone and nowhere to be found.

[ilyagr]

I see this too, some more details:

I tried sending a file from Mac OS running
https://github.com/schollz/croc to an Android phone with crocgui. It worked fine until the phone fell asleep. Then, the sending croc exited with the message:

Video...  71% |██████████████      | (359/504 MB, 690 kB/s) [8m26s:3m29s]2024/06/06 14:08:48 EOF

(note the EOF at the end).

When I woke up the phone, the crocgui app showed an error and there was no obvious way to resume.

I've tried re-sending the files again with the same code, and it started the upload from the beginning.