"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-backend/crates/cardclaws-api/tests/cards_test.rs

@@ -344,7 +344,8 @@ async fn demo_publish_creates_scannable_profile_with_welcome() {
                 "title": "Founder",
                 "links": [{ "label": "Site", "url": "https://cardclaws.com" }],
                 "welcomePrompt": "a calm forest at dawn",
-                "welcomeMessage": "Great to meet you"
+                "welcomeMessage": "Great to meet you",
+                "payload": { "kind": "code", "label": "20% off", "value": "CARD20" }
             })),
         )
         .await;
@@ -352,13 +353,17 @@ async fn demo_publish_creates_scannable_profile_with_welcome() {
     let handle = body["handle"].as_str().unwrap();
     assert!(body["profileUrl"].as_str().unwrap().ends_with(handle));
 
-    // The published card resolves publicly with the owner name + welcome.
+    // The published card resolves publicly with the owner name + welcome + payload.
     let (status, profile) = app
         .request("GET", &format!("/v1/cards/handle/{handle}"), None, None)
         .await;
     assert_eq!(status, StatusCode::OK);
     assert_eq!(profile["ownerDisplayName"], "Omar Sobh");
     assert_eq!(profile["welcome"]["message"], "Great to meet you");
+    assert_eq!(
+        profile["definition"]["profile"]["payload"]["value"],
+        "CARD20"
+    );
 }
 
 #[tokio::test]