🔒 Security: Fix pragma, zero-address validation, and ETH recovery

[canychan]

This PR addresses three low-severity security findings identified during an automated audit:

1. SWC-103: Floating Pragma

Files: contracts/src/Sandwich.sol, contracts/src/lib/SafeTransfer.sol

• Changed pragma solidity >=0.8.0; to pragma solidity 0.8.19;
• Pinning the compiler version ensures reproducible builds and prevents unexpected behavior from future compiler changes.

2. SWC-123: Missing Zero Address Validation

File: contracts/src/Sandwich.sol

• Added require(_owner != address(0), "Invalid owner"); in the constructor.
• Prevents accidental deployment with a zero address, which would permanently brick the contract.

3. SWC-105: ETH Locked

File: contracts/src/Sandwich.sol

• Added recoverETH() function with owner-only access control.
• Allows recovery of accidentally sent ETH that would otherwise be permanently locked in the contract.