feat: add ERC-721 completion certificate contract

Add CertificateNFT.sol with configurable mint fee, owner
withdrawal, and CertificateMinted event for tracking module
completions. Includes Foundry-based deployment instructions
for Base mainnet.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: beihaili

contracts/CertificateNFT.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
+import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
+import "@openzeppelin/contracts/access/Ownable.sol";
+
+/**
+ * @title Web3 Education Completion Certificate
+ * @notice ERC-721 NFT minted when a user completes a course module.
+ * @dev Mint fee is configurable by owner. Deploy on Base for low gas.
+ */
+contract CertificateNFT is ERC721, ERC721URIStorage, Ownable {
+    uint256 private _nextTokenId;
+    uint256 public mintFee;
+
+    event CertificateMinted(address indexed to, uint256 tokenId, string moduleId);
+
+    constructor(uint256 _mintFee)
+        ERC721("Web3 Education Certificate", "W3CERT")
+        Ownable(msg.sender)
+    {
+        mintFee = _mintFee;
+    }
+
+    function mint(string calldata tokenURI_, string calldata moduleId) external payable {
+        require(msg.value >= mintFee, "Insufficient mint fee");
+        uint256 tokenId = _nextTokenId++;
+        _safeMint(msg.sender, tokenId);
+        _setTokenURI(tokenId, tokenURI_);
+        emit CertificateMinted(msg.sender, tokenId, moduleId);
+    }
+
+    function setMintFee(uint256 _mintFee) external onlyOwner {
+        mintFee = _mintFee;
+    }
+
+    function withdraw() external onlyOwner {
+        payable(owner()).transfer(address(this).balance);
+    }
+
+    function tokenURI(uint256 tokenId) public view override(ERC721, ERC721URIStorage) returns (string memory) {
+        return super.tokenURI(tokenId);
+    }
+
+    function supportsInterface(bytes4 interfaceId) public view override(ERC721, ERC721URIStorage) returns (bool) {
+        return super.supportsInterface(interfaceId);
+    }
+}

contracts/README.md

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+# Smart Contracts
+
+## CertificateNFT
+
+ERC-721 completion certificate deployed on Base.
+
+### Prerequisites
+- [Foundry](https://book.getfoundry.sh/getting-started/installation)
+- Base mainnet ETH for gas (~$1-5)
+
+### Setup
+```bash
+cd contracts
+forge init --no-commit
+forge install OpenZeppelin/openzeppelin-contracts --no-commit
+```
+
+### Deploy
+```bash
+# Set mint fee to 0.002 ETH (~$5)
+forge create --rpc-url https://mainnet.base.org \
+  --private-key $PRIVATE_KEY \
+  CertificateNFT \
+  --constructor-args 2000000000000000
+
+# Verify on Basescan
+forge verify-contract $CONTRACT_ADDRESS CertificateNFT \
+  --chain base \
+  --constructor-args $(cast abi-encode "constructor(uint256)" 2000000000000000)
+```
+
+### After Deployment
+Update `src/features/nft/contractAbi.js` with the deployed contract address.