Adds a 'Cron' schedule type option to the scheduled task editor, allowing
users to create and edit cron-based schedules through the UI.
- Add cron expression input with inline validation (cron-parser)
- Show next 4 upcoming run times as a preview
- Provide clickable example chips (every 5min, hourly, Monday 9am, etc.)
- Preserve cron expressions when editing existing cron tasks
- Add cron.ts utility module for validation and next-run computation
- Add i18n keys across all 8 locale files
Closes#1586
Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>
