Adding accounts on KDE Plasma

### Problem

Currently, trying to add accounts on Plasma creates an "open with..." dialog, but there's no way to set up any accounts.

### Proposal

I assume it will require Plasma-specific changes that you might not be willing to do. But Mail has a lot of potential, as it's a much more modern client than others you can use on Plasma (like KMail)

### Prior Art (Optional)

_No response_