list-accounts: show source type

## Context

With IMAP support added in #193, a user can have both Gmail and IMAP sources. The `list-accounts` output doesn't distinguish between them.

## Proposal

Include the source type in `list-accounts` output:

```
you@gmail.com (gmail)
imaps://user@imap.mail.yahoo.com:993 (imap)
```

## References

- IMAP support: #193