feat: refine schedule notification permission flow

[jjoonleo]

Summary

• Refine schedule permission behavior so Android can keep notification fallback delivery when exact timing is denied
• Split user-facing copy between notifications and iOS AlarmKit alarms
• Add OS-specific schedule notification flow documentation and ADRs
• Align GitHub Actions Flutter version with the dependency graph required by sign_in_with_apple

Validation

• flutter pub get
• flutter analyze
• flutter test test/presentation/app/cubit/alarm_gate_cubit_test.dart test/presentation/alarm_allow/alarm_allow_screen_test.dart test/presentation/my_page/my_page_screen_test.dart