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/src/product/HarborConversation.tsx

@@ -0,0 +1,15 @@
+'use client';
+
+import { useState } from 'react';
+import { ConversationPanel, type Density } from './ConversationPanel.tsx';
+import { HarborStream } from './HarborStream.tsx';
+
+/** The Harbor Bakery conversation on its own, with its own detail level. */
+export function HarborConversation({ defaultDensity = 'steps' }: { defaultDensity?: Density }) {
+  const [density, setDensity] = useState<Density>(defaultDensity);
+  return (
+    <ConversationPanel density={density} onDensityChange={setDensity}>
+      <HarborStream density={density} />
+    </ConversationPanel>
+  );
+}