Add user-friendly scheduling for checks

[clone1018]

Summary

• Add a shared schedule picker for check creation and editing with common frequencies, local time controls, custom cron, and a next-three-runs preview.
• Store each check's time zone and evaluate schedules in that zone, including clear daylight saving behavior and local time display.
• Keep existing schedules on UTC and update the documentation.

Verification

• mix assets.build
• mix test — 307 passed, 15 excluded
• git diff --cached --check