fix: verify notification permission UX

[jjoonleo]

Closes #443
References #465

Summary

• Updates notification permission rationale and recovery copy to explicitly describe schedule preparation reminders in English and Korean.
• Adds a small permission gateway around the notification allow screen so granted, denied, request, and settings-recovery paths can be widget-tested.
• Adds [Release] Verify notification permission UX #443 implementation plan under plans/.

Verification

• flutter pub get
• flutter gen-l10n
• dart run build_runner build --delete-conflicting-outputs (current build_runner reports this flag is ignored, generation completed)
• dart format lib/presentation/notification_allow/screens/notification_allow_screen.dart lib/l10n/app_localizations.dart lib/l10n/app_localizations_en.dart lib/l10n/app_localizations_ko.dart test/presentation/notification_allow/notification_allow_screen_test.dart
• flutter test test/presentation/notification_allow/notification_allow_screen_test.dart
• flutter analyze
• flutter test

Track impact

• Completes the notification permission UX slice for [Release Track] Android permissions and alarm policy #465.
• Helps unblock Android alarm/notification QA [Release] QA alarm and notification flows on Android #457 after the remaining permission-policy sub-issues are complete.

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[github-actions]

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