feat: add shared marketplace, Account shield/unshield, and local QR

[JaDi03]

Summary

• Demo seeds plus optional Upstash Redis listings (local fallback when env is empty). Publish shows the claim key once; delete also tries the shared store.
• Account tab: own shielded balance on explicit read, Shield and Unshield on the same amount field. Unshield is a public withdraw to this wallet; the UI says to spread withdrawals over time. Relay timeouts warn that the tx can still land.
• Pay QR is generated in-browser (qrcode) so the pay URL never hits a third-party renderer. Basic response headers added.