Proton does not currently offer CalDAV or CardDAV services, however there is a highly modular platform used by many large companies that allows for simple integration into third party authentication services: Baikal, backed by sabre/dav.
Does Proton have a published API that could be leveraged for authentication? sabre/dav already has a plugin for IMAP, but of course that can’t put targeted to Proton. Many services offer access to their IDP these days, I’m wondering if there is a service like this from Proton as well?
Usually sync calendar and contacts.
Proto calendar does have an ics thing. Maybe you can use drive to backup your contacts
Not there. But would be great. At least a way to backup and recover your contacts to a new phone (both Android and iOS). That would be awesome.