Static pages, SEO, and PWA

Added

• Standalone About, FAQ, and How-To-Play pages ship with a shared stylesheet.
• The site gains SEO metadata, OpenGraph and Twitter previews, a PWA manifest, and a sitemap.

Changed

• The splash screen uses a static largest-contentful-paint image, and the favicon and OpenGraph image get lighter versions.
• Asset licensing is clarified under CC0 with a trademark disclaimer.

Fixed

• How-To-Play instructions are corrected, and errors handled by their caller stop double-toasting.