Inconsistent use of ReentrancyGuard

The fundRewards function lacks the nonReentrant modifier while other token-interacting functions (stakeTokens, closePosition) implement it. This inconsistency appears to be an oversight during the addition of this new function:

/**
 * New Function: Fund rewards manually
 * Allows owner to send extra tokens to the contract so users can claim their interest.
 */
function fundRewards(uint256 amount) external onlyOwner {
    // Missing nonReentrant modifier
}

While there is no immediate security risk due to trusted tokens, this inconsistency in security patterns could cause issues if the contract is upgraded or integrated with different tokens in the future.

Add the nonReentrant modifier to fundRewards for consistency with other token-interacting functions.