A RANGE event with a blank end date passed validateEventForm and reached
the backend, which 400s with a generic INVALID_DATE_RANGE mapped to "end
must not be before start" — wrong for a missing end date, and shown only as
a top-of-form alert. Validate it before the API call and surface a dedicated
event_editor_end_date_required message on the end-date field via a new
DatePrecisionField endDateError prop (defaults '', so the document form is
unchanged).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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