Let users type or slide period, grace, and interval on create

[nihey]

Summary

• New check Period, Grace, and Interval now share Timeout's seconds field plus slider, so heartbeat and probe times can be typed instead of slider-only.
• The duration field keeps an out-of-range draft while typing and clamps on blur, so a plan floor like 300s does not block entering 600.

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

• Open New check as Heartbeat: type Period and Grace, confirm slider follows, then drag the slider
• Type a Period below the plan floor, blur, confirm it clamps to the floor
• Switch to HTTP: type Interval and Timeout
• Cron heartbeat still has a typeable Grace field