fix: correct clone URL, remove stale ngrok ref, align explorer URL

[skyc1e]

Summary

• README: Update clone URL from akelani-circle/fintech-starter to circlefin/arc-fintech and the cd directory accordingly
• README: Remove the dangling "Replace your-ngrok-url … from step 4" instruction — step 4 is "Start the development server" and no ngrok step exists
• gateway-sdk.ts: Replace explorer.arc.testnet.circle.com (connection refused) with testnet.arcscan.app to match block-explorers.ts

Verification

• https://testnet.arcscan.app — resolves (Blockscout-based Arc Testnet explorer)
• https://explorer.arc.testnet.circle.com — ECONNREFUSED
• block-explorers.ts already uses testnet.arcscan.app for ARC-TESTNET

[Sertug17]

+1 All three fixes verified locally on a fresh clone:

• The clone URL akelani-circle/fintech-starter returns 404;
  circlefin/arc-fintech is correct.
• The "your-ngrok-url ... from step 4" reference is dangling
  step 4 is npm run dev, no ngrok step exists in the README.
• Reproduced the explorer URL issue:
  curl -I https://explorer.arc.testnet.circle.com/ → HTTP 000 (connection failed)
  curl -I https://testnet.arcscan.app/ → HTTP 200 OK
  And lib/constants/block-explorers.ts already maps ARC-TESTNET
  to testnet.arcscan.app, so this aligns cross-file usage.
  Nice catch on the inconsistency.

For context: the same incorrect akelani-circle/... clone URL pattern
also exists in arc-escrow's README
and might be worth a follow-up PR there.

Thanks @skyc1e would be great to see this merged 🙏