Implement Local Storage with Data Export/Import #98

Title: Add Data Persistence with JSON Export and Import Functionality

Problem Statement

User data is lost when the browser cache is cleared or when switching devices. Users need the ability to backup their data and transfer it between devices or browsers.

Proposed Solution

Create a comprehensive data management system with:

• Automatic localStorage sync
• Export data as JSON file
• Import data from JSON file
• Data validation on import
• Clear all data functionality
• Backup reminders