Unit Converter

# Unit Converter PRD

## Overview
Build a unit converter application that supports multiple measurement categories with real-time bi-directional conversion and a clean, responsive interface.

## Requirements
- Support categories: length, weight, temperature, volume, area, speed, time, data storage
- At least 6 units per category (e.g., length: mm, cm, m, km, inches, feet, yards, miles)
- Bi-directional conversion — type in either input field to convert
- Real-time conversion as the user types (no submit button needed)
- Swap units button to quickly reverse the conversion direction
- Display the conversion formula used (e.g., "1 km = 0.621371 miles")
- Save favorite and recent conversions to localStorage
- Search and filter units within a category
- Copy result to clipboard with one click
- Responsive layout that works on mobile and desktop
- Handle edge cases: negative temperatures are valid, negative lengths are not
- Decimal precision selector (0-8 decimal places)
- Conversion history showing recent conversions with timestamps

## Tech Stack
- HTML, CSS, JavaScript (vanilla or any framework)
- No unit conversion libraries — define conversion factors yourself

## Scoring Criteria
- **Functional (40%)**: Conversions are accurate, real-time updating works, all categories present
- **Quality (20%)**: Correct math, handles edge cases, clean and readable code
- **Fidelity (25%)**: All features present including favorites, search, history, precision selector
- **Speed (15%)**: Time bonus