Summary: Division by zero in vested amount calculation if vesting period has zero duration.

Vulnerability Details: If vesting_start_time equals vesting_end_time, vesting_duration becomes zero, causing a division by zero in _calculate_vested_amount

Impact: High. Prevents all claims, rendering the contract unusable

Tools Used

Recommendations : Add checks in the constructor to ensure vesting_end_time > vesting_start_time