fix(android): improve exact alarm permission UX

[jjoonleo]

Summary

• Separates Android exact-alarm permission checks from notification permission so the native alarm channel reports the exact alarm state directly.
• Adds precise alarm rationale/settings recovery copy to the schedule alarm toggle flow.
• Documents Android exact-alarm QA scenarios and adds reconciliation coverage for denied exact alarm permission with and without notification fallback.

Closes #444
References #465

Verification

• dart run build_runner build --delete-conflicting-outputs
• flutter test test/domain/use-cases/reconcile_alarms_use_case_test.dart test/core/services/alarm_scheduler_service_test.dart
• flutter analyze

Unblocks

• Helps unblock [Release] QA alarm and notification flows on Android #457 after this PR is merged alongside [Release] Audit Android manifest permissions for release #442, [Release] Verify notification permission UX #443, and [Release] Build a signed release Android App Bundle #452 prerequisites.

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