FEAT: Complete JMAP backend support

[andrinoff]

Is your feature request related to a problem?

A JMAP backend scaffold exists but isn't a full alternative to IMAP.

Describe the solution you'd like

Flesh out the JMAP backend (mailbox/email/identity, push via EventSource) as a first-class backend alongside IMAP.

Describe alternatives you've considered

IMAP only.

Additional context

See backend/jmap and the backend factory backend/factory.go.

[floatpanebot]

Thanks for the suggestion!

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