web: the facet column is a wall below 62rem #237

Below 62rem the facet column stacks after the list as three ragged inline-block groups of unbounded length (style.css, the max-width: 62rem block). On the issue, merge request and build lists the block is taller than the content it filters.

Two causes:

• Layout. Spec rule 7 stacks the column after the content. That holds for a five-item group and not for a twenty-five item one.
• Content. listFacets emits every repository label whatever its count, so the merge request list lists 25 labels of which 24 have a zero count and link to an empty list. buildFacets lists every ref distinctRefs saw with no cap; topicFacets already caps at 20.

Fix: drop zero-count labels and milestones, cap the branch group, and make the column a closed disclosure before the list at narrow widths.