Split ScheduleFormBloc orchestration seams

[jjoonleo]

Summary

• Add Schedule, Preparation, Default Preparation, Custom Preparation, and Preparation Mode glossary terms to align schedule-form language with Product Usage Event terminology.
• Introduce narrow schedule-form seams for draft loading, create/update submission orchestration, and schedule create analytics payload construction.
• Remove direct AuthBloc and ProductUsageEventTracker dependencies from ScheduleFormBloc; create flow now passes current-user spare time as explicit event input.
• Simplify schedule form Bloc/widget tests and add focused domain tests for defaults, unchanged/changed preparation persistence, create analytics, and failure handling.

Validation

• dart run build_runner build --delete-conflicting-outputs (succeeds; build_runner warns this option is ignored by the installed version)
• flutter analyze
• flutter test

Closes #539