AccountSyncManager now starts a _JmapAccountSync loop for JMAP accounts
alongside the existing _AccountSync for IMAP accounts.
_JmapAccountSync:
- Syncs mailboxes then emails for each known mailbox per cycle.
- Polls every 30 seconds (no IDLE for JMAP; EventSource deferred).
- Reuses the same exponential backoff (5–300 s) on failure.
- stop() interrupts the poll wait immediately via a Completer.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
