Issue 5: Complete Pomodoro Timer Implementation

## Overview
Implement a fully functional Pomodoro timer with session tracking and notifications.

## Tasks
- [ ] Implement timer countdown functionality
- [ ] Add session type switching (focus, short break, long break)
- [ ] Implement session completion tracking
- [ ] Add browser notifications
- [ ] Implement session history
- [ ] Add timer controls (start, pause, reset, skip)
- [ ] Implement auto-switch between sessions
- [ ] Add timer settings customization
- [ ] Implement session statistics
- [ ] Add sound notifications

## Acceptance Criteria
- [ ] Timer counts down correctly
- [ ] Session switching works automatically
- [ ] Notifications appear when sessions end
- [ ] Session history is tracked and displayed
- [ ] Timer controls work properly
- [ ] Settings can be customized
- [ ] Statistics are accurate

## Technical Details
- Use `setInterval` for countdown
- Implement browser notifications API
- Store session data in Supabase
- Add audio feedback for session completion
```

---

No files or tests are named. Start by locating the existing Pomodoro timer entry point in the React application and the Supabase session-data integration, then review how notifications and settings are handled. Done means the listed controls, session transitions, persistence, notifications, history, and statistics satisfy the acceptance criteria.

Written by the indexing model from the issue text.