Create a demo showing HTTP 402 payment-gated access to a premium article using testnet4.

• Article as JSON-LD on the pod
• ACL with PaymentCondition (amount, currency, chain)
• Server returns 402 with payment details
• User deposits testnet4 sats, balance checked, article served
• Simple frontend pane to handle the 402 flow

Builds on #239 (HTTP 402 conditions) and the existing /pay/ deposit infrastructure.