[codex] Fix schedule date-time validation and picker cancel flow

[jjoonleo]

Summary

• Fix schedule date display to use the month token instead of minutes.
• Show schedule spare time as total minutes.
• Keep the date/time Next button disabled while async overlap checks run and after picker cancellations when no value was selected.
• Add widget regression coverage for overlap validation and repeated date/time picker cancellation.

Validation

• flutter analyze
• flutter test test/presentation/schedule_create/components/schedule_multi_page_form_test.dart

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