Restructures HorizontalScrollRow so paired arrows sit in a flex header
beside an optional section title (instead of absolute-positioned at the
row's left/right edges). Adds margin-right: -1rem to the row so the
scroller's items extend to the viewport's right edge, escaping Shell's
p-4 padding. The header bar keeps its right gutter so titles align with
the rest of the page content.
Home page passes 'title' to the first row of each section; subsequent
rows in multi-row sections render arrow-only headers.
Reusable horizontally-scrolling row with left/right arrow buttons,
scroll-boundary disabling, snap scrolling, and hover-reveal arrows.
Used by M6a home page (Task 17).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
bvandeusen