fix(ui): hide customs seals link in union mode

Summary

Fixes #311. In union mode the seals lookup is keyed on the instance ISO code (e.g. EU), which has no entry in ITDB, the modal always shows "Seals information not found in ITDB". Hides the "Show related seals/stamps information" button and skips SealsInfoModal when SYSTEM_UNION_CODE is set.

Testing

Added a unit test, the change is a pure render conditional, and reproducing it live needs a TIR transport with an operation. The test mocks useUnion and asserts the button + modal render in standalone mode and are absent in union mode. Both cases pass.