A frontend dir under plugins/ with no manifest.json is a CORE feature, not a
per-site plugin - applications is one (backend is shopdb/core/api/applications.py,
nav is advertised as core in dashboard.py). stage-frontend.mjs treated it like
a plugin and dropped it under SITE_PLUGINS, so a lean site showed the core
Applications nav item but had no route for it -> blank page. Now manifest-less
frontends always stage regardless of SITE_PLUGINS; SITE_PLUGINS selection applies
only to real plugins. CI lean-build job asserts ApplicationsList ships in a lean
bundle. Found while testing a live machines+printers lean site.
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