Rework campaign detail stats: hide on drafts, consistent rates, grouped

[alexeygrigorev]

Part of UX epic #67. From the operator UX audit (finding 6, HIGH).

Problem

Campaign detail shows ~14 stat tiles; on a draft every tile is 0 (noise). Rates use inconsistent denominators ("Delivered 4 / 133.3%", "Skipped 1 / 14.3%"). (campaign_detail.html:61-82, campaign_stats())

Ask

For drafts, hide the Stats block and show only the Queue Preview. For sent campaigns, group into Delivery (sent/delivered/bounced/complained) and Engagement (opens/clicks/unsubs) with a single consistent denominator; move Queued/Processed/Send speed into a Send-progress subsection.

Acceptance

• Draft campaigns show no all-zero stat grid.
• Rates use one consistent denominator; no >100% artifacts.
• Delivery vs Engagement grouped; send-progress separated.
• Tests + lint pass.