Group featured course offerings under "Recommended" - #74641

CourseOffering#is_featured has existed for some time -- column, levelbuilder
checkbox, serialization, seeding -- but its only remaining consumer was a
tiebreak in curriculumRecommender.js that never fires, because
summarize_for_catalog does not emit the field. Give the flag the two behaviors
it was meant to have.

Quick assign groups a featured offering under a "Recommended" header in place
of the header it was assigned, not in addition to it, so each offering is
listed exactly once. "Recommended" sorts ahead of every other header,
including "Favorites" and "Year Long". In the grade-band views the group nests
inside the offering's curriculum_type and so appears at the top of that
column; the HoC, HoAI and PL views get a single group at the top of the first
column.

The curriculum catalog at /catalog lists featured offerings first,
alphabetical by display name within the featured and non-featured groups.
Ordering stays server-side, so the catalog filters narrow the list without
reordering it.

The blank-header and blank-curriculum_type guards are unchanged. Featuring an
offering promotes it within quick assign; it does not publish one that
curriculum staff have not yet given a header.

No migration and no config-file churn. 27 offerings already set is_featured,
22 of which reach the catalog, and they change position on deploy.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>