Fix page layout width and section navigation

[sanity]

What changed

• widen long-form prose from 36rem to 42rem and pair it with an 18px/1.6 reading treatment
• give section landing pages the full site shell instead of the article-width container
• render child-page summaries as responsive cards
• remove the redundant nested .section wrapper that doubled page padding, especially on mobile
• use theme-aware link color in section cards

Why

The default list and single layouts applied the same 576px article container to nearly every page. That kept line length readable, but made landing and index pages look stranded inside large gutters and left useful page width unused.

Validation

• hugo --minify --gc — 123 pages built successfully
• Playwright representative screenshots at mobile, tablet, laptop, and desktop widths in light and dark modes
• Playwright all-page audit: 94 sitemap URLs at 375, 768, 1024, 1366, and 1920px (470 combinations), with zero horizontal overflow or new browser errors

A pre-existing mobile canvas error in the July 2026 Reveal presentation reproduces on the deployed site and is unrelated to this change.