Add Pomodoro Timer Feature

Add a Pomodoro timer to help students focus on studying.

**Task:**
- Create a new PomodoroTimer class
- Implement 25-minute work sessions and 5-minute breaks
- Add CLI command to start timer
- Show countdown in terminal
- Play sound or notification when session ends (optional)

**Expected Usage:**
```bash
python main.py pomodoro --sessions 4
```