Follow-up: validate EmailBuilder import graph (dangling refs / cycles)

[patoperpetua]

Context

PR #142 import path validates each block via Zod (EditorConfigurationSchema) but does not verify that every childrenIds entry exists or that container references are acyclic. A document with missing IDs can pass validation and throw when rendering.

Goal

• Reject dangling childrenIds references before accepting imported JSON
• Reject cycles in container/column hierarchies
• Add failing tests for both cases, then fix

Related: #142
Parent: #138