Lean build: always ship core (manifest-less) frontends

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.

.github/workflows/ci.yml

@@ -82,6 +82,11 @@ jobs:
               echo "FAIL: chosen-plugin code '$code' missing from the lean bundle"
               exit 1; }
           done
+          # Core frontends (no manifest, e.g. applications) must ship in EVERY
+          # build regardless of SITE_PLUGINS, or a lean site loses a core page.
+          grep -rqoh "ApplicationsList" "$assets"/*.js || {
+            echo "FAIL: core page 'ApplicationsList' missing from the lean bundle"
+            exit 1; }
           test -d /tmp/leansite/plugins/machines
           test ! -d /tmp/leansite/plugins/printedparts
           echo "lean build verified: only chosen plugins present"