`.sendme` folders not cleaned up after Ctrl+C

When cancelling a transfer, the temporary folders still stay around:

# on the sender side
$ sendme send something
^C
$ ls -a | grep .sendme
.sendme-send-(redacted)

# on the receiver side
$ sendme receive (redacted)
^C
$ ls -a | grep .sendme
.sendme-get-(redacted)

From my understanding, these folders process resumes, as deleting them will restart a transfer from scratch. However, they accumulate quickly. When testing out sendme, I had 4 of them in my Downloads folder already without knowing!

Here's a few ideas on how to solve this:

• Maybe move these folders somewhere central, so they don't get littered around your current working directory. Maybe one of the XDG directories, like $XDG_CACHE_HOME/sendme.
• Clean up resume data older than a certain period when sendme is next ran.
  • Alternatively, have a sendme clean command (or maybe both at once!). I don't like the idea of command-only as it's easy to forget to do.
  • The expiration period should probably be configurable somewhere, but I don't think sendme has any persistent config mechanisms yet.
• Delete the temporary folder when a transfer is cancelled voluntarily. I'd want to resume a transfer if my network drops out, but I wouldn't want to resume it most times I cancel it myself (e.g. I decide I don't want the file, or I'll be sending something else instead).
  • I'm not sold on this idea, because this might not be desired behavior, because sometimes you want to resume a transfer when cancelling it (e.g. someone needs to go do something else for a short period). This could be solved with a command line argument, or maybe an interactive hotkey in the terminal (like how there's c to copy to clipboard), but it would probably be easy to forget.

This may also be related to #47? I noticed some weirdness cancelling a few times.

No files or tests are named. Start by tracing the send and receive command paths for cancellation and resume data, then review issue #47; a complete change would need an agreed cleanup policy, including how intentional cancellation differs from interrupted transfers.