[Contract] Stablecoin (USDC on Stellar) support + multi-currency display

[Gbangbolaoluwagbemiga]

User story

Freelancers and clients want to transact in a stablecoin (USDC on Stellar) to avoid XLM price
volatility, with values shown in a familiar currency.

What needs to be done

• Add Circle USDC (Stellar SAC) to the token whitelist; verify the full lifecycle works for a
  non-native token: create_escrow → fund → approve_milestone payout → refund paths
• Confirm do_transfer (refund_system.rs) and the SAC token::Client path handle USDC decimals
• Frontend: token selector in CreatePage; display amounts with correct symbol/decimals
• Optional: show an approximate fiat value via a price source

Acceptance criteria

• A full escrow can be created, funded, released, and refunded in USDC on testnet
• UI shows the correct token symbol and decimal precision everywhere
• Non-whitelisted tokens are rejected

Relevant files

contracts/secureflow/src/refund_system.rs, escrow_core.rs (is_whitelisted_token), src/pages/CreatePage.tsx

Complements #6 (per-milestone token override).