[FE-01] Implement Stellar blockchain payment checkout flow

[devwums]

Summary

Implements the Stellar blockchain payment checkout flow for the platform, enabling customers to pay with XLM, USDC, and BITE loyalty tokens.

Changes

• Added rontend/components/checkout/StellarCheckout.tsx — a fully accessible checkout component that:
  • Displays a line-item order summary with total
  • Lets users select their preferred Stellar asset (XLM / USDC / BITE)
  • Validates the payer's Stellar public key format (G…, 56 chars) before submission
  • Calls the payment backend (placeholder wired to /payments/stellar/initiate) and renders the resulting transaction hash with a Stellar Explorer deep-link
  • Handles loading, error, and success states

closes #269