Add SEP-24 fiat onramp/offramp flow

[MJ-RWA]

Complexity: High — 200 pts

Summary

SEP-24 is Stellar's standard for interactive fiat deposits and withdrawals via anchors. This issue integrates a SEP-24 flow so users (especially Nigerian users) can deposit NGN and receive USDC — or withdraw USDC back to NGN — directly within StellarFlow using a SEP-24 compatible anchor.

Files to Create/Modify

• Create: src/lib/sep24.ts
• Create: src/pages/OnrampPage.tsx
• Modify: src/App.tsx — add /onramp route
• Modify: src/pages/DashboardPage.tsx — add Buy/Sell quick action

Implementation Notes

sep24.ts

Three functions:

1. fetchAnchorToml(homeDomain) — fetch /.well-known/stellar.toml, regex out TRANSFER_SERVER_SEP0024 and WEB_AUTH_ENDPOINT
2. sep10Authenticate(authServer, address, signTx) — GET challenge transaction, sign with Freighter via the passed signTx callback, POST signed tx, return JWT
3. initiateDeposit(transferServer, jwt, assetCode) — POST to /transactions/deposit/interactive with Bearer auth, returns { url, id }
4. pollTransaction(transferServer, jwt, txId) — GET /transaction?id= to check status

OnrampPage

• Hardcode testanchor.stellar.org for testnet
• Deposit tab + Withdraw tab, asset selector (USDC)
• "Start Deposit"/"Start Withdrawal" opens the anchor interactive URL (iframe or new window)
• Poll /transaction?id= every 3 seconds, show status progression: pending_user_transfer_start → pending_anchor → completed

Acceptance Criteria

• SEP-1 TOML parsed to discover anchor endpoints
• SEP-10 JWT obtained by signing challenge with Freighter
• SEP-24 interactive deposit URL opens correctly
• Transaction status polled every 3 seconds and displayed
• Works with testanchor.stellar.org on testnet
• Withdrawal flow implemented alongside deposit
• Graceful error if anchor is unavailable

Screen Recording Requirements (using testanchor.stellar.org)

1. Open Onramp page, click Start Deposit
2. Anchor interactive iframe/modal opens
3. Complete the deposit flow in the anchor UI
4. Status updates: pending → completed
5. USDC balance increases on the Dashboard