ux(frontend/recs): enlarge cell-expand chevron + move it to the checkbox column

[cristim]

Problem

The expand/collapse chevron on multi-variant rows (the "▶" / "▼"
button in recommendations.ts:buildVariantRowMarkup /
buildGroupHeaderMarkup) is too small to be a comfortable click
target and sits in the resource-type cell rather than the checkbox
column. Subitems (variants) align with the checkbox column visually,
so the chevron should anchor there too — that's the natural axis the
user's eye follows when scanning the cell.

Acceptance criteria

• Chevron rendered in the checkbox column on cell-summary rows
  (where the master checkbox would be on a flat row), with adequate
  padding to be a 24×24+ click target.
• Variant sub-rows keep their own per-row checkbox in the same
  column (no overlap).
• Larger glyph (CSS font-size + line-height bump or SVG icon).
• Keyboard activation still works (Enter/Space) and the
  aria-expanded toggle stays correct.
• Visual regression test or screenshot snapshot covering the
  cell-summary + expanded states.