Add Cron Expression Parser tool

[GhDj]

Summary

• Add new Cron Parser tool at /tools/cron
• Parse and validate cron expressions with human-readable explanations
• Display field breakdown (minute, hour, day of month, month, day of week)
• Calculate and show next 5 scheduled run times
• Include 12 common patterns library for quick reference
• Add syntax reference panel for cron special characters
• Client-side only implementation using Alpine.js

Features

• Expression parsing: Validates syntax and explains each field
• Human-readable output: "At 9:00 AM, Monday through Friday"
• Next runs: Shows upcoming 5 execution times
• Common patterns: Quick-insert for typical schedules
• Syntax reference: Field ranges and special characters

Test plan

• All 273 tests passing
• Verify cron page loads at /tools/cron
• Test expression parsing with various patterns
• Verify next run time calculations
• Check dark/light theme support

[GhDj]

Merged via cherry-pick into develop