Flows: email steps, tags, and delivery windows

A big round of upgrades to the flow builder.

The flow editor with the action palette

Send email from a flow: a new email action with the full email composer, so automations can send designed emails, not just texts.
First-class tags: add or remove contact tags as flow steps, and use tags in triggers and conditions, making it easy to chain flows and build audiences from automation behavior.
Delivery windows: cap the hours a flow is allowed to send each day (with weekdays-only and apply-to-all-days shortcuts) so a 2 AM trigger becomes a 9 AM message.
Per-step recipients: every action node on an active flow now shows who is currently at that step, with a drill-in panel for debugging why someone did or did not get a message.

See flow actions and logic and timing for the full picture.