Decide reply send-from account and how sent replies re-enter the conversation

[Hohnik]

Part of #12

Question

Because a Conversation can merge messages from more than one connected account (spec #7, user story 17), sending a reply needs a "From" account chosen from among the user's connected accounts. Decide the rule (e.g. the account the most recent inbound message in the conversation arrived through).

Separately, fetch_recent_messages (src/mailchat/sync/imap_client.py) only reads the INBOX folder, so a sent reply won't reappear via the normal poll unless something else surfaces it. Decide whether the send path persists the outbound Message directly at send time, or whether sync also needs to poll the Sent folder.

Record both answers as an update to #11's acceptance criteria.

[Hohnik]

Resolution

From-account selection: reply from the connected account associated with the conversation's most recent Message row (inbound or outbound) — no explicit picker.

Re-entry mechanism: no direct-persist-at-send. sync_account polls both INBOX and a per-account Sent folder; the Sent folder is user-selected during account connect from a real folder list (reusing imap_client.list_folders), stored as Account.sent_folder. Chosen over auto-detection (SPECIAL-USE/guess-list) because every other server-specific setting in this app is already manually configured at connect time, and over direct-persist-at-send alone because that only captures replies sent through MailChat — replies sent via another client on the same account (a real near-term case, since nothing forces exclusive use of MailChat) would otherwise never appear.

Freshness: a successful send triggers an immediate sync_account run for that one account, so the reply shows up without waiting for the next scheduled poll — same normalize/dedup pipeline as everything else, no separate code path.

Message.direction: new "inbound"/"outbound" column, set by sync from the source folder (INBOX → inbound, Sent folder → outbound), not inferred from headers. This also closes a latent gap in #10: its acceptance criteria already assumed a From/To split by direction (from #14) without ever specifying how direction is known.

Scope note: this decision touches three tickets, not just #11 —

• Connect and store a mail account #8 (Connect and store a mail account) reopened, amended with the Sent-folder picker step.
• Group messages into person+subject conversations #10 (Group messages into person+subject conversations) amended to reference Message.direction.
• Reply within a conversation #11 (Reply within a conversation) amended with the from-account rule, Sent-based re-entry, and immediate-poll trigger.