Threading and background-indexer fixes.
Fixed
- Sent messages no longer orphaned from their conversation. A message sent through MailFlow was filed as its own separate thread instead of joining the conversation it replied to, showing as a duplicate "shadow" beside the real thread (most visibly for mail between accounts on the same domain). Sent copies now thread via their References/In-Reply-To chain like a normal sync, and existing orphaned copies self-heal on the next folder sync. (#378)
- Snippet indexer now finishes instead of looping. Messages that can't produce a snippet (body in an unfetched MIME part, HTML-only, attachment/calendar-only, or empty) were re-processed on every batch, so the backlog never drained and the indexer ran indefinitely. Each message is now marked once attempted and skipped thereafter, and a wider set of body parts is fetched so more messages get a real snippet. (#379)