[codex] Restore preparation timers from action events

[jjoonleo]

Summary

• Persist timed preparation runs with startedAt and user Preparation Action Events instead of relying on suspended timer ticks.
• Rebuild ongoing preparation progress from start time, skip actions, and the current time on app resume/re-entry.
• Preserve restoration across backend preparation freeze when step IDs change but the preparation shape is unchanged.
• Clear local timed preparation state when schedules are finished, deleted, stale, or structurally changed.

Root Cause

Dart timer ticks can stop while the phone sleeps, and the old restoration path depended on elapsed snapshots. After schedule start, backend frozen preparation can also change step IDs/fingerprints, which caused local run snapshots and skip actions to be discarded.

Validation

• dart run build_runner build --delete-conflicting-outputs
• flutter analyze
• flutter test test/presentation/app/bloc/schedule/schedule_bloc_test.dart
• flutter test
• git diff --check