[codex] fix default preparation copy on schedule edit

[jjoonleo]

Summary

• Preserve the schedule's original preparation mode when opening the edit form.
• When a DEFAULT schedule's preparation is edited, copy the steps with fresh UUIDs before calling PUT /schedules/{scheduleId}/preparations.
• Keep existing step IDs unchanged for CUSTOM schedules.

Root Cause

DEFAULT schedules render user default preparation rows from /users/preparations. The edit flow was sending those default preparationId values back to the schedule-specific update endpoint, which creates schedule-owned preparation rows. Reusing user-default IDs can conflict with existing preparation_user rows and trigger PREPARATION_STEP_ID_CONFLICT.

Validation

• flutter test test/presentation/schedule_create/bloc/schedule_form_bloc_test.dart
• flutter analyze
• git diff --check