fix(docs): stretch the embeddable architecture page's capability panel

The standalone embed page (embed/architecture.html, for iframing the architecture diagram on external sites) reuses landing.css unmodified, but its own wrapper CSS set align-items: flex-start on the flex row containing the diagram and the capabilities panel. That let .architecture-capabilities size to its own content height instead of stretching to match the (taller) diagram, so .capability-detail's flex: 1 1 0 had no extra height to grow into and the hover/pinned description box was cut short.

The docs landing page (index.md) does not have this problem because its equivalent layout is a CSS grid, which stretches items by default. Switch the embed page to the flexbox default (align-items: stretch) to match.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com