"Payload drop" scan experience — hand over a link or code on scan

Experience #8: a published card can carry a payload that's shown prominently on
scan — a primary link CTA, or a discount/access code with tap-to-copy.

- Backend: demo publish accepts an optional payload {kind, label, value},
  stored in definition.profile.payload (flows through the public profile). Test
  extended to assert it. Gate green (120 tests).
- Web ([handle].astro): renders a "link" payload as a big CTA button and a
  "code" payload as a dashed chip with copy-on-tap. astro check clean.
- Mobile (publish.tsx): payload kind chips (None/Link/Code) + label + value,
  sent with publishToWeb.

Verified live: publish with a code payload -> profile shows a copyable CARD20
chip; link payload -> CTA button.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

cardclaws-mobile/src/api/ai.ts

@@ -56,6 +56,7 @@ export interface PublishPayload {
   links: { label: string; url: string }[];
   welcomePrompt?: string;
   welcomeMessage?: string;
+  payload?: { kind: "link" | "code"; label: string; value: string };
 }
 
 /** Publish a demo card to the web (no auth) → a real, scannable profile URL. */