Cron Expression Builder

# Cron Expression Builder PRD

## Overview
Build a visual cron expression builder that generates cron strings from interactive selectors, provides human-readable descriptions, previews upcoming execution times, and supports both standard and extended cron formats.

## Requirements
- Visual selectors for each cron field: minute, hour, day-of-month, month, and day-of-week
- Support for standard 5-field cron and extended 6-field cron with seconds
- Human-readable description generated from the cron expression (e.g., "Every Monday at 3:00 PM")
- Preview of the next 10 execution times based on the current expression
- Real-time cron string output that updates as selectors change
- Manual cron string input field with parsing back into the visual selectors
- Preset templates for common patterns: every minute, hourly, daily, weekly, monthly, yearly
- Validate cron expressions and display error messages for invalid input
- Highlight invalid fields visually with error styling
- Copy the cron string to clipboard with one click
- Support special cron characters: *, /, -, comma, L, W, and #
- Examples panel showing common cron patterns with descriptions
- Timezone selector for adjusting the execution time preview
- Dark and light theme toggle
- Responsive layout for desktop and mobile
- Persist recent expressions to localStorage

## Tech Stack
- HTML, CSS, JavaScript (vanilla or any framework)
- No cron parsing libraries — build the parser, validator, and scheduler yourself

## Scoring Criteria
- **Functional (40%)**: Visual selectors produce correct cron strings, descriptions are accurate, execution previews are correct
- **Quality (20%)**: Clean code, handles edge cases (invalid expressions, leap years), bidirectional sync between input and selectors
- **Fidelity (25%)**: All features implemented including extended cron, presets, timezone selector, examples panel, theme toggle
- **Speed (15%)**: Time bonus