[FE-01] Implement Stellar blockchain payment checkout flow

[Leothosine]

Problem

The platform markets cryptocurrency payments (XLM, USDC, BITE loyalty tokens) as a core feature, but there is zero checkout or payment implementation. frontend/app/(public)/cart/page.tsx is actually a workspace check-in page unrelated to restaurant ordering. The cartStore and AddToCartModal components exist but lead nowhere - orders are dispatched via a WhatsApp link generator, not a payment system. No PaymentMethod, Transaction, or LoyaltyToken types exist anywhere in the frontend.

Proposed Solution

Build a /checkout page with:

1. Order summary (items, quantities, subtotal, tax, total)
2. Payment method selector: card | xlm | usdc | whatsapp
3. Stellar wallet connection using the Freighter browser extension API
4. Transaction submission to Stellar testnet via @stellar/stellar-sdk
5. Confirmation screen displaying the on-chain txHash linking to Stellar Expert Explorer

Add types: PaymentMethod, Transaction { id, amount, currency, txHash, status, createdAt }, LoyaltyToken { balance, symbol, earnRate }

Acceptance Criteria

• User can connect a Freighter wallet and see their XLM/USDC balance
• Selecting XLM or USDC and confirming creates a real Stellar testnet transaction
• Confirmation page displays the txHash as a link to https://stellar.expert/explorer/testnet/tx/{hash}
• WhatsApp fallback still works for non-crypto users
• Cart clears after successful payment
• Failed transaction shows an error with the Stellar error code

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. Thank you!

[Senorespecial]

I would really like to work on this issue, please assign.