feat: BSC testnet support — deploy escrow on Chapel and wire USDT/USDC

[AgrimTawani]

Overview

Add BSC (BNB Smart Chain) as a live chain on CryptoBazaar. The escrow contract is pure EVM/Solidity — the same CryptoBazaarEscrow.sol we already run on Polygon Amoy deploys unchanged to BSC Chapel testnet. This is mostly infrastructure + frontend wiring work.

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Why BSC

BSC is the most popular chain for USDT volume in India. A large portion of Indian crypto users hold BNB and use PancakeSwap. Gas fees on BSC are ~$0.01–0.05 USD, comparable to Polygon, making it viable for small trades.

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Current State

• CryptoBazaarEscrow.sol is already deployed on Polygon Amoy (testnet).
• The sell page (frontend/src/app/marketplace/sell/page.tsx) hardcodes amoyChain (chainId 80002), USDC_ADDR, and ESCROW_ADDR — no chain selection exists yet.
• The trade page (frontend/src/app/marketplace/[id]/page.tsx) also hardcodes amoyChain and the Polygon escrow contract.
• The DB schema and API already have chain: "BSC" as a valid enum value.
• isEvm in the trade page correctly includes BSC: order.chain === "POLYGON" || order.chain === "BSC".

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Implementation Plan

1 — Deploy contract to BSC Chapel (chainId 97)

BSC Chapel testnet is fully EVM-compatible. No contract changes needed.

# in contracts/evm/
forge script script/Deploy.s.sol \
  --rpc-url https://data-seed-prebsc-1-s1.binance.org:8545 \
  --broadcast \
  --verify

• Get test BNB from: https://testnet.bnbchain.org/faucet-smart
• Find or deploy a test USDT token on Chapel (BSC doesn't have an official test USDT; deploy a mintable ERC-20 mock and call setWhitelist on the escrow).
• Save the deployed addresses to .env as NEXT_PUBLIC_ESCROW_BSC_ADDRESS and NEXT_PUBLIC_CHAPEL_USDT_ADDRESS.

2 — Update sell page for multi-chain

Currently the sell page hardcodes Polygon. Need to:

• Add a Chain selector (Polygon / BSC) in the form.
• Based on selected chain, resolve the correct escrowContract, tokenContract, and chain object (defineChain(80002) for Polygon Amoy, defineChain(97) for Chapel).
• Pass the selected chain to the DB via the existing POST /api/orders body field chain.

// rough shape
const chainConfig = {
  POLYGON: {
    chain: defineChain(80002),
    escrowAddr: process.env.NEXT_PUBLIC_ESCROW_POLYGON_ADDRESS,
    tokenAddr: process.env.NEXT_PUBLIC_AMOY_USDC_ADDRESS,
    asset: "USDC",
  },
  BSC: {
    chain: defineChain(97),
    escrowAddr: process.env.NEXT_PUBLIC_ESCROW_BSC_ADDRESS,
    tokenAddr: process.env.NEXT_PUBLIC_CHAPEL_USDT_ADDRESS,
    asset: "USDT",
  },
};

3 — Update trade page for multi-chain

The trade page currently hardcodes amoyChain and the Polygon escrow contract. It needs to resolve both from order.chain:

const chainCfg = order.chain === "BSC"
  ? { chain: defineChain(97), escrowAddr: process.env.NEXT_PUBLIC_ESCROW_BSC_ADDRESS }
  : { chain: defineChain(80002), escrowAddr: process.env.NEXT_PUBLIC_ESCROW_POLYGON_ADDRESS };

const escrowContract = getContract({
  client: thirdwebClient,
  chain: chainCfg.chain,
  address: chainCfg.escrowAddr as `0x${string}`,
});

All prepareContractCall calls already use escrowContract as a reference so they'll automatically pick up the right chain once the contract object is dynamic.

4 — ThirdWeb wallet — no changes needed

ThirdWeb v5 supports BSC natively. defineChain(97) is enough. The user's MetaMask will prompt a network switch if they're on the wrong chain. The existing AutoConnect and walletOk guard work as-is.

5 — Vercel env vars

Add to Vercel (production + preview):

NEXT_PUBLIC_ESCROW_BSC_ADDRESS=0x...
NEXT_PUBLIC_CHAPEL_USDT_ADDRESS=0x...

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Test Procedure

1. Get Chapel BNB from the faucet.
2. Mint test USDT to the seller wallet (call mint() on the mock token).
3. Open /marketplace/sell, select BSC, post an order — confirm MetaMask switches to Chapel (chainId 97), two popups fire (approve + createOrder).
4. Open order in a second browser session as buyer — confirm lockOrder fires on Chapel.
5. Submit payment proof — confirm markPaid fires on Chapel.
6. Confirm payment as seller — confirm confirmPayment fires and USDT lands in buyer wallet.
7. Verify the ✓ on-chain ↗ link in the marketplace goes to the BSC testnet explorer (testnet.bscscan.com).

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Files to Touch

File	Change
contracts/evm/script/Deploy.s.sol	Add Chapel deploy target
frontend/src/app/marketplace/sell/page.tsx	Chain selector, dynamic contract resolution
frontend/src/app/marketplace/[id]/page.tsx	Dynamic contract resolution from order.chain
frontend/src/lib/explorer.ts	Add BSC Chapel explorer URL mapping
.env.example	Document new BSC env vars