Read-state sync to platforms (phone unread parity, read receipts) — per-platform opt-in

[MaxGhenis]

Context

Reading a conversation in OpenMessage only updates the local store (/api/conversations/read → MarkConversationRead). Nothing is pushed to the source platform, which means:

• Your phone keeps showing the thread unread after you read it in OpenMessage (and Google Messages kept re-asserting its stale unread flag into the app until Harden HTTP API; fix stale-snapshot unread clobber; contacts query pushdown #31's snapshot guard).
• Senders never see read receipts from messages you read in OpenMessage, even when you have read receipts enabled on the platform.

Why this is a product decision, not just a bug

Pushing read state is recipient-visible on some platforms (WhatsApp blue ticks; Signal read receipts when enabled). Some users specifically value reading in OpenMessage without triggering those. Others expect read state to sync like every first-party client. Both are legitimate.

Proposal

A per-platform "sync read state" setting (default on for Google Messages — phone⇄app consistency, not recipient-visible for SMS; default off for WhatsApp/Signal until explicitly enabled):

• Google Messages: libgm exposes mark-read; wire /api/conversations/read to it.
• WhatsApp: whatsmeow MarkRead (sends receipts per account settings).
• Signal: signal-cli sendReceipt --type read (respects the account's read-receipt setting; cleanest after Replace Signal receive polling with a long-lived signal-cli jsonRpc process #36's jsonRpc migration).

Surface the toggles on the Platforms cards.