Phase 4: wire demo lead forms to live n8n webhook

- demo-tier-a / demo-tier-b: forms POST JSON to /webhook/lead-intake
  (hidden client_id + source, named fields), async submit with
  success/error toast.
- lead-intake webhook: enable CORS (allowedOrigins=*) so the static
  demos (different origin) can submit; re-export n8n/lead-intake.json.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

n8n/lead-intake.json

@@ -6,7 +6,9 @@
         "httpMethod": "POST",
         "path": "lead-intake",
         "responseMode": "onReceived",
-        "options": {}
+        "options": {
+          "allowedOrigins": "*"
+        }
       },
       "id": "f60ee339-9809-4962-9c5d-68ad9a5ec452",
       "name": "Webhook",