Forward email to external recipient

[mageaustralia]

Summary

End-to-end "forward this email" feature with internal-thread filtering so back-and-forth with a forwarded recipient never leaks into customer-facing replies.

Backend

• New forwarded_to field on the send-message request. When set, the message is routed to those addresses instead of the conversation contact; CC/BCC pass through unchanged so an agent can loop in internal teammates on the forward.
• At SMTP send: forwards strip In-Reply-To/References so the recipient sees a fresh thread, and prepend Fwd: to the subject (idempotent).
• Activity-note records who forwarded to whom.
• New is-source-id-from-forward SQL + intake hook: any incoming message whose In-Reply-To/References point at a previously-sent forward (or an already-tagged reply chain) gets meta.from_forward = true. Empty source-IDs are filtered before the lookup; the from_forward check uses jsonb @> '...' containment so it never throws on unexpected meta shapes.

Frontend

• Per-message Forward button on email-channel, non-private bubbles.
• ReplyBox listens for the forward event (via onMounted/onBeforeUnmount with matching .off), switches into a new forward mode, pre-quotes the original message, and clears recipient fields.
• The Forward tab is only visible while in forward mode (no orphan tab in normal use).
• Switching conversations or tabs out of forward resets the pre-quoted body and recipient fields so a stale forward can't be accidentally sent into a different conversation.
• Forward-related bubbles (outgoing forward + incoming reply-to-forward) share the private-note bubble color so the internal thread groups visually distinct from customer-facing messages.
• Small "Forwarded to:" badge on outgoing forwards and "Reply to forward (internal)" badge on tagged incoming.

Pairs especially well with #289 (CID inline-image embed) — without that PR, forwarded inline images use signed URLs that expire ~1 hour later. This PR works without #289 but with that lifetime caveat.

Test plan

• Open an email conversation, hover any non-private message → small Forward link appears under the bubble
• Click Forward → reply box switches to forward mode, third tab visible, TO/CC/BCC empty, editor pre-populated with ---- Forwarded message ---- From: ... Date: ... + the original body
• Type an external recipient in TO + optional CC, send → outgoing email has Fwd: <subject>, no In-Reply-To, recipients include CC; bubble shows in cream bg-private color with "Forwarded to: ..." badge
• When the forwarded recipient replies (via IMAP): their reply lands in the same conversation, bubble is also in cream, with "Reply to forward (internal)" badge underneath
• Switch to that conversation's reply tab and reply to the customer → the auto-quoted thread does not include the internal forward exchange
• Switch conversations mid-forward → forward state resets, no stale pre-quoted body in the new conversation
• Switch tabs from Forward to Reply mid-forward → editor resets, recipient fields clear