Let users switch duration fields between seconds, minutes, and hours

[nihey]

Summary

• Period, grace, and interval now open in the coarsest whole unit (for example 5 min instead of 300 seconds) with an s/min/hr toggle beside the number.
• Switching units converts the stored duration; timeout stays seconds-only because its range is 1–60s. Create and edit both use the shared control.

Validation

• Added or updated tests for behavior changes
• Ran relevant tests and checks
• Updated relevant documentation
• Confirmed no production data, credentials, or populated .env files are included

Checklist

• Conventional commit subject used
• Scope is focused and ready for review

Test plan

• New heartbeat check: Period shows 5 min; type 10; switch to seconds and confirm 600
• Switch Period to hours, then back to minutes, without the duration jumping
• HTTP Interval uses minutes; Timeout stays in seconds with no unit toggle
• Edit an existing heartbeat and change Grace in minutes