Add Subscription smart contract playground

[jin-Pi]

Summary

• Replaces the Order Apple/Lemon Pie payment demo on the Shop page with a SubscriptionPanel UI for exercising window.Pi.SmartContract.Subscription methods (registerService, subscribe, process, getSubscription, getSubscriptionReservation, getSubscriberSubscriptions, etc.)
• Adds corresponding SmartContract.Subscription type declarations to window.d.ts
• Note: frontend/index.html currently points the Pi SDK to http://localhost:9000/v2-production.js (SDK v3.0) instead of the CDN — a local dev override left in intentionally per author's request, should be reviewed before merge
• usePayments.ts retains commented-out debug experiments used while building the panel — also left as-is per author's request

Test plan

• Restore SDK src to CDN (or confirm intentional) before merging
• Open Shop page inside Pi Browser and verify SubscriptionPanel renders and each action calls through to window.Pi.SmartContract.Subscription
• Clean up leftover debug code in usePayments.ts if not needed

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