feat(document): surface end-before-start inline on the date form (#678)

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>

frontend/src/lib/shared/errors.ts

@@ -8,6 +8,7 @@ export type ErrorCode =
 	| 'PERSON_NOT_FOUND'
 	| 'ALIAS_NOT_FOUND'
 	| 'INVALID_PERSON_TYPE'
+	| 'INVALID_DATE_RANGE'
 	| 'DOCUMENT_NOT_FOUND'
 	| 'DOCUMENT_NO_FILE'
 	| 'FILE_NOT_FOUND'
@@ -87,6 +88,8 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_alias_not_found();
 		case 'INVALID_PERSON_TYPE':
 			return m.error_invalid_person_type();
+		case 'INVALID_DATE_RANGE':
+			return m.error_invalid_date_range();
 		case 'DOCUMENT_NOT_FOUND':
 			return m.error_document_not_found();
 		case 'DOCUMENT_NO_FILE':