Track how long each mailbox takes to sync and display it in the
sync log expanded view (e.g. "2 new · 5 up-to-date · 1.3s").
- Add optional `duration` field to `MailboxSyncStats`
- Capture per-mailbox start/end time in both IMAP and JMAP sync loops
- Store as `duration_ms` in `sync_log_mailboxes` (schema v30 migration)
- Read back and reconstruct `Duration` in repository
- Show timing alongside fetch/skip counts in per-mailbox breakdown
- Extract `_fmtDuration` helper, reuse for the existing total duration
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>