Consider using `block.timestamp` instead of local time in `calculateStartWindow`

[cedoor]

The calculateStartWindow function in the Enclave JS SDK currently relies on the local system timestamp to compute the start window. This can lead to inconsistencies between client-side calculations and on-chain timing, since Enclave contracts use block.timestamp as the canonical reference.

Using local time can cause race conditions or invalid window predictions in environments with clock drift or when simulating networks.