fix: improve notification system reliability and logging

- Prevent duplicate notifications using DispatchWorkItem for cancellation
- Clear legacy UNCalendarNotificationTrigger notifications on app startup
- Add multiple cleanup retry attempts (1s, 3s, 5s, 10s after init)
- Clear legacy notifications when app becomes active
- Add detailed logging for notification debugging:
  - notification_cleanup_start/done
  - notification_start_target_computed
  - notification_timer_approaching
  - notification_sent
- Remove paytick_ prefix from log filenames for better sorting

Co-authored-by: Cursor <cursoragent@cursor.com>