FEAT: JMAP push (EventSource) for new-mail notifications

[andrinoff]

Describe the feature

Implement the JMAP push channel (RFC 8620 §7.3) using EventSource so JMAP accounts (Fastmail) receive new-mail events in real time without polling.

Why this matters

JMAP is the modern push transport. Fastmail users currently have to fall back to short polling, which is both slow and rate-limited.

Notes

• The go-jmap library exposes a push.Stream. Wire it into the same IdleUpdate channel as IMAP IDLE.
• Reconnect on EventSource disconnect with the same backoff strategy as IDLE.