Add Due Dates, Filters, and LocalStorage Persistence with Notifications

### Description

Enhance the Task Manager app by adding due date tracking, task filtering (All / Active / Completed), and localStorage persistence to retain tasks across sessions.
Additionally, implement optional alerts or browser notifications when a task’s due date approaches, improving usability and user engagement.

### Proposed Features

- Due Date Field: Add date input to tasks and display in the list.
- LocalStorage: Save and load tasks automatically across sessions.
- Task Filters: Add All / Active / Completed filters and sort by nearest due date.
- Notifications: Show alerts or browser notifications for upcoming or overdue tasks.
- UI Enhancements: Add color indicators for urgency (red = overdue, orange = soon) and optional bell icon for notifications.

### Implementation Details

- Update AddTask.js, TaskList.js, and main state logic (App.js or Redux store).
- Use useEffect hooks to persist data and trigger due-date checks.
- Use the Web Notifications API for native desktop alerts.
- Update README.md to document new features.