Simplify /lessons filter UI — replace button-heavy facets with dropdowns / cap button count

[jt14den]

Part of #194.

Current state

LessonFilter.tsx's renderPromotedFacet renders two facets as always-visible button rows instead of collapsed controls:

• Audience ("Who are the learners?") — 8 values
• Topic ("What subject?") — 13 values (src/data/topics.ts)

13 topic buttons in one row is very likely the "cluttered with buttons" feedback from the meeting.

Ask (2026-08-04 WG meeting)

• Convert the button rows back to a dropdown/select control, or
• Cap visible buttons at ~5 with an overflow/"more" affordance

Reference point

The WG called out the filter UI at ucospo.net/education/lessons/ (org main, pre-fork) as the better pattern — worth a side-by-side comparison against the current fork before picking an approach.

Related

#194, facets-vs-sections issue