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/lock.svg

@@ -0,0 +1,5 @@
+<svg preserveAspectRatio="none" overflow="visible" style="display: block;" width="18.7368" height="18.7368" viewBox="0 0 18.7368 18.7368" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g id="Icon/lock">
+<path id="Vector" d="M5.46491 8.58772V5.46491C5.46491 4.42964 5.87617 3.43676 6.60822 2.70472C7.34027 1.97267 8.33315 1.5614 9.36842 1.5614C10.4037 1.5614 11.3966 1.97267 12.1286 2.70472C12.8607 3.43676 13.2719 4.42964 13.2719 5.46491V8.58772M3.90351 8.58772H14.8333C15.2474 8.58772 15.6446 8.75222 15.9374 9.04504C16.2302 9.33786 16.3947 9.73501 16.3947 10.1491V15.614C16.3947 16.0281 16.2302 16.4253 15.9374 16.7181C15.6446 17.0109 15.2474 17.1754 14.8333 17.1754H3.90351C3.4894 17.1754 3.09225 17.0109 2.79943 16.7181C2.50661 16.4253 2.34211 16.0281 2.34211 15.614V10.1491C2.34211 9.73501 2.50661 9.33786 2.79943 9.04504C3.09225 8.75222 3.4894 8.58772 3.90351 8.58772Z" stroke="#8F8B80" stroke-width="1.40526" stroke-linecap="round" stroke-linejoin="round"/>
+</g>
+</svg>