Empty phases save as drafts, not errors

Estimates now correctly persist as drafts when phases have no scope or job-level rules.

Bug Fixes

Previously, creating an estimate for a phase with no user-entered scope and no job-level rules (like remediation-abatement) would fail with a "No line items assembled" error. The estimate is now correctly saved as a draft — it's a legitimate state that users encounter before adding scope to their work areas. The per-work-area UI already highlights "No scope" to make the next step clear.