Return alarm screen home when no active schedule exists

[jjoonleo]

Alarm No-Schedule Fallback Plan

Goal

Ensure /alarmScreen returns to /home when opened while the schedule state is already notExists, instead of showing a loading spinner indefinitely.

Context

• SRD: if no active schedule exists, alarm should fall back through notExists handling and navigate home.
• Current AlarmScreen listener only navigates when the previous state was not notExists and the current state becomes notExists.
• If /alarmScreen is opened while ScheduleBloc.state.status is already notExists, the listener does not fire and the builder shows the loading scaffold.
• This can happen after a stale notification, deleted schedule, or already-ended schedule.

Decisions

• Treat an already-notExists state on alarm entry as terminal for the alarm route.
• Navigate home after build, not synchronously inside build.
• Preserve finish-result navigation when _navigateAfterFinish is pending.

Steps

1. Add a post-frame fallback in AlarmScreen when current state is notExists and no finish navigation is pending.
2. Keep the existing listener for transitions into notExists.
3. Add a widget test that pumps /alarmScreen with initial ScheduleState.notExists() and expects navigation to /home.
4. Add a stale-notification-style test where subscription emits null while already notExists.

Validation

• Run flutter test test/presentation/alarm/screens/preparation_flow_widget_test.dart.
• Confirm finish flow still navigates to /earlyLate when pending finish data exists.

Open Questions

• None.