Make Pages the complete theme showcase

[lilabrooks]

What changed

• turns the GitHub Pages root into the complete Compact Theme showcase
• demonstrates typography, navigation, cards, buttons, responsive media, code labels, token colours, copy controls, data-no-copy, light and dark modes, reduced motion, accessibility utilities, and no-JavaScript fallback
• adds the dynamic W3C validation badge and live showcase link to the README
• fixes code-containing cards at narrow widths with min-width: 0
• expands static and Playwright coverage for showcase sections and behaviors
• updates all macOS and Linux visual baselines

Validation

• W3C Nu: zero messages
• npm test: 30 passed, 18 intentional skips
• Axe: zero detectable violations in Chromium, Firefox, and WebKit
• responsive checks: 320px through 3440px
• no-JavaScript fallback: passed in all three engines
• Pages artifact: exactly 11 allowlisted files
• macOS and pinned Linux visual baselines: passed