Add an endBeforeStart $derived to WhoWhenSection (lexicographic ISO compare,
no Date object) that renders an inline error on the end-date field —
border-red-400, aria-invalid, aria-describedby, and a #end-date-error <p>
inside the existing aria-live region — with a ⚠ glyph so the cue is not
colour-alone (WCAG 1.4.1). Save is not disabled; the server stays the gate.
Wire ErrorCode INVALID_DATE_RANGE through errors.ts getErrorMessage and add
the single key error_invalid_date_range to de/en/es, so the same translated
string is used inline (client) and via getErrorMessage (server fallback).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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