Habit Tracker

# Habit Tracker PRD

## Overview
Build a habit tracking application that lets users define habits, check them off daily, visualize streaks with a GitHub-style heatmap, and view detailed statistics.

## Requirements
- Add habits with a name, color, and frequency (daily or weekly)
- Check off habits for each day with a single click
- Display current streak and longest streak per habit
- Calendar heatmap view (GitHub contribution graph style) showing completion density
- Weekly completion percentage for each habit
- Habit categories or groups for organization (e.g., Health, Work, Learning)
- Set daily reminders using the Notification API (with permission request)
- Archive completed or inactive habits (hidden from main view but data preserved)
- Undo today's check-in for a habit
- Add optional notes per day per habit
- Statistics dashboard with weekly and monthly trend charts
- Drag to reorder habits in the list
- Export all data as JSON file download
- Persist all data to localStorage
- Responsive mobile-friendly design

## Tech Stack
- HTML, CSS, JavaScript (vanilla or any framework)
- Canvas or SVG for heatmap visualization

## Scoring Criteria
- **Functional (40%)**: Habit creation works, check-off works, streaks calculated correctly
- **Quality (20%)**: Accurate streak logic, smooth UI interactions, clean code
- **Fidelity (25%)**: All features present including heatmap, statistics, reminders, export
- **Speed (15%)**: Time bonus