feat: EIP-712 structured signing + on-chain signature verification for mintOriginal

[pradhyum6144]

Summary

• Adds on-chain EIP-712 signature verification to mintOriginal() in LensMintERC1155.sol, fixing the core vulnerability where the signature parameter was stored but never verified — allowing anyone with an active device address to mint with a fake signature string like "0xsignature123"
• Adds replay protection via mapping(bytes32 => bool) usedImageHashes (prevents the same image from being minted twice) and per-device nonces (prevents signature reuse)
• Updates the backend (web3Service.js) to use ethers.Wallet.signTypedData() with the matching EIP-712 domain, and updates the ABI to the new mintOriginal(address, string, bytes32, uint256, uint8, bytes32, bytes32) signature

What changed

Contract (LensMintERC1155.sol)

• Inherits OpenZeppelin's EIP712("LensMintERC1155", "1")
• Defines MINT_ORIGINAL_TYPEHASH for the structured type: MintOriginal(address to, string ipfsHash, bytes32 imageHash, uint256 maxEditions, uint256 nonce)
• mintOriginal() now accepts (uint8 v, bytes32 r, bytes32 s) instead of string _signature, reconstructs the EIP-712 digest on-chain, and verifies ECDSA.recover(digest, v, r, s) == msg.sender
• imageHash changed from string to bytes32 for gas efficiency and proper cryptographic comparison
• Exposes domainSeparator() view function for off-chain signing compatibility

Tests (LensMintEIP712.t.sol — 15 new tests)

Test	What it verifies
testMintOriginalWithValidSignature	Happy path — valid EIP-712 sig mints successfully
testNonceIncrementsAfterMint	Nonce increments per device after each mint
testMintTwoDifferentImages	Sequential mints with different images work
testEditionMintStillWorks	Edition minting is unaffected by the change
testRevertOnDuplicateImageHash	Same image cannot be minted twice
testRevertOnFakeSignature	Garbage v/r/s values are rejected
testRevertOnWrongSignerKey	Signature from wrong private key is rejected
testRevertOnSignatureFromDifferentDevice	Cross-device signature replay blocked
testRevertOnTamperedParams	Changed recipient detected via digest mismatch
testRevertOnTamperedIpfsHash	Changed IPFS CID detected
testRevertOnWrongNonce	Stale nonce signature rejected
testRevertOnUnregisteredDevice	Unregistered device still blocked
testRevertOnDeactivatedDevice	Inactive device still blocked
testDomainSeparatorIsConsistent	Domain separator is deterministic
testUsedImageHashTracking	usedImageHashes mapping updates correctly

Backend (web3Service.js)

• New signMintOriginal() method builds the EIP-712 domain + types and calls wallet.signTypedData()
• mintOriginal() now signs before calling the contract with (v, r, s) params
• ABI updated to match new contract interface

Config (foundry.toml)

• evm_version changed from paris → cancun (fixes mcopy instruction errors with latest OpenZeppelin, related to forge test fails with mcopy instruction error due to EVM version mismatch #10)

Why EIP-712 over EIP-191

	EIP-191 (personal_sign)	EIP-712 (this PR)
What user sees	Raw hex blob	Structured fields: to, ipfsHash, imageHash
Domain binding	None	Tied to contract address + chain ID
Cross-chain safety	Same sig works on any chain	Domain separator prevents cross-chain replay
Replay protection	Must be added separately	Nonce is part of the signed struct

Closes #2

Test plan

• All 18 Foundry tests pass (forge test — 15 new + 3 updated existing)
• Deploy to Sepolia testnet and verify minting flow end-to-end
• Verify hardware camera app signing integration (Python sign_hash → EIP-712 format)