feat(web): marketing Home page and the product component library (ADR 0023)

Product UI components are built once at design size from the Figma file and reused by the marketing mockups and, later, the app. Text contrast tokens now meet WCAG AA in both themes, with a test. Pricing stays hidden until decided.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014SdXCbrN5PtGExkpvMaMHx

web/public/icons/sparkles.svg

@@ -0,0 +1,6 @@
+<svg preserveAspectRatio="none" overflow="visible" style="display: block;" width="20" height="20" viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g id="Icon/sparkles">
+<path id="Vector" d="M10 2.5L11.5833 6.75L15.8333 8.33333L11.5833 9.91667L10 14.1667L8.41667 9.91667L4.16667 8.33333L8.41667 6.75L10 2.5Z" stroke="#6B675E" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path id="Vector_2" d="M15.8333 12.5L16.5 14.3333L18.3333 15L16.5 15.6667L15.8333 17.5L15.1667 15.6667L13.3333 15L15.1667 14.3333L15.8333 12.5Z" stroke="#6B675E" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</g>
+</svg>