Add Manual Time Reminders Feature

My mobile app is already live and stable; the only missing piece is a reminder system. I want users to pick any date and time, save it, and receive a push notification at the chosen moment. The flow is simple: open the “Reminders” screen, tap “Add,” select the date-picker and time-picker, optionally choose a notification sound, and confirm. No location triggers, no automatic suggestions—just straightforward, user-defined, time-based alerts.

Key points I need you to cover:
• Implement local push notifications for both foreground and background states.
• Provide a small selector so the user can pick from the notification sounds I will supply.
• Ensure reminders persist after the device restarts and fire accurately even if the app is closed.
• Keep the UI in line with the current design.

At hand-off I expect the updated project files, clear setup notes, and a brief test plan so I can verify that scheduling, editing, and deleting reminders all work flawlessly.