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/home.svg

@@ -0,0 +1,6 @@
+<svg preserveAspectRatio="none" overflow="visible" style="display: block;" width="18" height="18" viewBox="0 0 18 18" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g id="Icon/home">
+<path id="Vector" d="M2.25 6.75L9 1.5L15.75 6.75V15C15.75 15.3978 15.592 15.7794 15.3107 16.0607C15.0294 16.342 14.6478 16.5 14.25 16.5H3.75C3.35218 16.5 2.97064 16.342 2.68934 16.0607C2.40804 15.7794 2.25 15.3978 2.25 15V6.75Z" stroke="#6B675E" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path id="Vector_2" d="M6.75 16.5V9H11.25V16.5" stroke="#6B675E" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</g>
+</svg>