Use custom errors instead of string revert messages

The contract uses string messages in require statements throughout the code:

require(_stakingToken != address(0), "Staking token address cannot be zero");
require(_nftContract != address(0), "NFT contract address cannot be zero");
require(msg.sender == owner, "Only owner can call this function");
// ... more require statements with strings

Custom errors, introduced in Solidity 0.8.4, are more gas-efficient and provide better error handling capabilities:

• Save deployment gas (no need to store error strings).
• Save runtime gas (cheaper than require with strings).
• Allow passing parameters for better error details.
• Provide clearer error handling in the frontend.

Replace require statements with custom errors:

error ZeroAddress(string parameter);
error Unauthorized(address caller, string role);
error InvalidAmount(uint amount, string reason);