Support ENABLE command (RFC 5161) #473

Summary

Implement RFC 5161 ENABLE command, which allows clients to activate optional IMAP extensions (like CONDSTORE and QRESYNC) for the session.

Motivation

iOS Mail sends ENABLE CONDSTORE QRESYNC immediately after authentication. ENABLE is a prerequisite for advertising and using CONDSTORE and QRESYNC capabilities. Without ENABLE support, clients cannot activate these extensions.

Scope

• Advertise ENABLE in CAPABILITY response
• Accept ENABLE <extension-list> command in authenticated state
• Return * ENABLED <extension-list> untagged response listing actually-enabled extensions
• Track enabled extensions per session
• Initially may enable nothing (respond with empty ENABLED), but provide the framework for CONDSTORE/QRESYNC activation

References

• RFC 5161
• Prerequisite for: CONDSTORE (Support CONDSTORE - RFC4551 #419), QRESYNC