Summary

Adds "Source unit" field

Plus formatting

Test Plan

Additional Information

For users: How does this change affect me?

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Summary by cubic

Add a “Source unit” field to alert values so authors can specify the metric’s original unit used in expressions, avoiding confusion with UI-scaled units.

• New Features
  • Added value.units_placeholder to the schema (default “-”), shown as “Source unit” and used before UI scaling.
  • Integrated unitsPlaceholder UI widget with help text.
  • Updated field order in the Value section to include Source unit alongside calculation, units, and update_every.
  • Minor UI layout tweaks (e.g., update_every column span).

^{Written for commit d2e28c0. Summary will update automatically on new commits.}