* admin: remove Scheduler Settings cards, make Next Run full-width
Remove the two "Scheduler Settings" placeholder cards from the plugin
UI (overview page and scheduler tab). They only contained a text note
saying detection intervals are configured per job type, which is
self-evident from the per-job-type settings form.
Make the "Next Run" card full-width on the overview page since it no
longer shares a row with the removed card.
* plugin UI: promote Next Run to top summary card row
Move "Next Run" from a standalone card into the top row alongside
Workers, Active Jobs, and Activities as a compact stat card.
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