Rename campaign send action and add a send confirmation

[alexeygrigorev]

Part of UX epic #67. From the operator UX audit (finding 4, HIGH — trust).

Problem

The button that commits a broadcast to a whole audience is labeled "Snapshot and queue" (internal term) and fires an immediate POST with no confirmation. (campaign_detail.html:32-37, mailing/views.py:436-452)

Ask

Rename to "Queue send" / "Send campaign" and add a confirmation that echoes the recipient estimate ("Queue send to ~N recipients?").

Acceptance

• Action uses plain language, not "snapshot".
• Confirmation step with recipient estimate before queueing.
• Tests + lint pass.