fix: read x402 payment response header

[peterxing]

Summary

• decode the x402 v2 PAYMENT-RESPONSE header on successful paid responses in the browser dApp
• fall back to the existing JSON body payment object for compatibility
• teach the Node demo client to read/log the settlement response from the header too

Why

Issue #3 calls out that the spec-compliant settlement channel is the PAYMENT-RESPONSE HTTP header. The resource server on the current POC branch already emits it; this patch makes both example clients consume it instead of relying only on the convenience JSON body.

Verification

• git diff --check
• node --check client.js
• local Base64 payment-response decode smoke: PAYMENT_RESPONSE_DECODE_OK
• npm run lint --prefix dapp -- --file components/X402Fetch.tsx
• npm run build --prefix dapp

Note: direct tsc --noEmit -p dapp/tsconfig.json is currently blocked by pre-existing test/mock type errors outside this patch (dapp/__mocks__/hathorCoreAPI.ts, dapp/__tests__/unit/hathorRPC.test.ts, etc.). The Next production build passed.

Refs #3