Let users know that cron schedule is UTC

[taylordowns2000]

Adds a small "All times in UTC" note to the cron trigger's edit step and read-only panel, since schedules always run in UTC but the UI never said so. It reuses the panels' existing muted hint style, and adds an optional hint prop to ReadOnlyField to show it there. Closes #5247.

Validation steps

1. Open the cron scheduler panel and check it out! 👀

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[github-actions]

Security Review ✅

• S0 (project scoping): N/A, this is a frontend-only UI change adding a "All times in UTC" label to cron trigger panels — no queries, data access, or project-scoped resources are touched.
• S1 (authorization): N/A, no new web-layer actions, controllers, LiveView events, or policy modifications; only presentational React components changed.
• S2 (audit trail): N/A, no mutations or writes to project/instance configuration resources — the diff is purely a cosmetic hint on display components.

[taylordowns2000]

@lmac-1 , @elias-ba , the HEX warning (lint failure) is unrelated to this branch.