Refactor/separate frontend from backend

[Oscar-SR]

This PR reorganizes the repository structure by separating the frontend and backend code into distinct directories:

device-activity-tracker/
├── backend/
│   └── src/
│       ├── tracker.ts         # WhatsApp RTT analysis logic
│       ├── signal-tracker.ts  # Signal RTT analysis logic
│       ├── server.ts          # Backend API server (both platforms)
│       └── index.ts           # CLI interface
└── frontend/                  # React web interface

Changes include:

• Moved all backend source files from the root directory to backend/src/
• Renamed the React application directory from client/ to frontend/
• Updated README.md to reflect the new project structure

No functional changes were made; this is purely a refactor to improve project organization. All existing imports and configurations have been adjusted accordingly.

This sets the foundation for future development and makes it easier to manage the backend and frontend independently.