[FR] Add Native Calendar & Task Alerts/Reminders for Desktop and Mobile Apps

### Description

AppFlowy should support native calendar and task notifications/reminders across desktop (Windows/macOS) and mobile applications.

Users should be able to create alerts for:
- Tasks
- Calendar events
- Deadlines
- Recurring reminders
- Time-sensitive projects
- Follow-ups and scheduled workflows

Notifications should behave like modern productivity/calendar applications with:
- Push notifications on mobile
- Native desktop notifications
- Custom reminder timing (5 min, 1 hour, 1 day, etc.)
- Recurring alerts
- Snooze functionality
- Persistent reminder support
- Notification center/history
- Optional sound and badge support

Currently, AppFlowy lacks the real-time reminder functionality expected from modern productivity platforms. Adding this would make task and calendar management significantly more practical for daily use.

### Impact

This feature would greatly improve AppFlowy’s usability as a true productivity and task management platform.

Without reminders and alerts, users must rely on external calendar apps or manually remember deadlines, reducing workflow efficiency and increasing mental overhead.

Native reminders would:
- Improve task completion rates
- Reduce missed deadlines
- Increase daily engagement
- Make calendars/tasks actually actionable
- Reduce dependence on external apps
- Improve mobile workflow adoption
- Make AppFlowy more competitive with ClickUp, Notion, Todoist, and other productivity tools

Reliable reminders are considered a core expectation in modern productivity software and are essential for serious daily workflow management.

### Additional Context

Potential enhancements:
- Cross-device sync for reminders
- Smart reminder suggestions
- Time zone-aware notifications
- AI-generated reminder recommendations
- Calendar integrations
- Quiet hours / focus mode support
- Notification actions (complete, snooze, reschedule)
- Widget and lock-screen reminder support on mobile

This feature would significantly strengthen AppFlowy’s position as a complete productivity workspace rather than just an organizational tool.