Move CoPlan administration into the engine

[HamptonMakes]

What changed and why

CoPlan admin pages lived in the demo host, so other hosts could not use them. Move shared ActiveAdmin registrations into the engine and mount them at /_/admin. Hosts can add their own app/admin pages in the same navigation.

The dashboard now shows notification intent and delivery outcomes for the last 7 days, recent failures, and a count of distinct Slack messages. One grouped Slack post counts once even when it covers several notifications. Admin access uses the host's CoPlan::Authentication user and its admin? role.

Evidence

Dark theme:

Light theme:

Testing

• Added a JavaScript-enabled system spec for menu navigation to the dashboard and delivery history.
• Added request specs for authorization, resources, menu visibility, and grouped Slack counts.
• Ran bundle exec rspec (2259 examples, 0 failures) before the final browser spec; ran bundle exec rspec spec/system/admin_spec.rb (1 example, 0 failures) after it.
• Opened the dashboard in light and dark themes in Chrome.
• Attached screenshots of both themes.

The companion Square host change is required to use this engine revision in production.

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-25T14:14:25.516603Z	e336aa8	Draft marked ready

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