fix: reject zero viewport training records

[alan747271363-art]

Summary

• tighten the generator-network record schema so innerWidth, innerHeight, clientWidth, and clientHeight must be positive
• add focused schema tests covering zero-value rejection and positive-value acceptance

Context

This follows the review feedback on #540: rejecting invalid screen dimensions in the training record schema prevents zero viewport values from entering the model in the first place, instead of normalizing generated fingerprints at runtime.

@algora-pbc /claim #6

[barjin]

Closing in favour of #544