feat(examples): add subscription paywall (Next.js) example (#354)

[Ipramking]

Summary

Implements #354 — Subscription paywall (Next.js): a new examples/subscription-paywall/ app demonstrating a recurring on-chain subscription paywall built on invisible-wallet-sdk.

How the subscription works

The subscription is a Soroban token allowance from the user's wallet contract to a merchant address, with an expiry:

Action	Call	Meaning
Unlock	wallet.approve(feePayer, merchant, token, price, expiry)	Authorize the merchant to pull up to price until expiry — the recurring payment authorization, confirmed with the user's passkey.
Status	get_allowance(merchant, token) (read-only simulation)	Subscription is active while the wallet holds a positive, unexpired allowance to the merchant.

Because the status is a read-only simulation it works on the client (useInvisibleWallet().getAllowance) and the server (lib/subscription.ts) alike.

Routes

Route	Access	Notes
/	Free	Landing + passkey wallet creation
/paywall	Free	On-chain status + Unlock button
/premium	Paid	Gated by middleware.ts; server component re-verifies from chain
/api/subscription	—	Node route that reads status from chain

middleware.ts matches /premium/:path*, reads the wallet cookie, asks the Node /api/subscription route (which reads the on-chain allowance), and redirects to /paywall when inactive — keeping the Stellar SDK out of the Edge runtime. It fails closed on read errors.

Acceptance criteria

• ✅ Free vs paid routes — / + /paywall open; /premium gated by middleware + a server-side re-check.
• ✅ Subscription status read from chain — get_allowance simulated against the wallet contract (server in lib/subscription.ts, client via the SDK).

Verification

Built against a locally-compiled sdk/dist. tsc --noEmit, next lint, and next build all pass (6 routes generated). Follows the conventions of the existing examples/nextjs.

Set NEXT_PUBLIC_MERCHANT_ADDRESS in .env.local to run — see the example README.

Closes #354

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