fix(dates): make DAY precision locale-aware in formatDocumentDate

DAY precision routed through formatDate() which hard-coded de-DE, so an
en/es reader saw the German month name ("24. Dezember 1943"). Route DAY
through Intl.DateTimeFormat(locale, …) like the other branches, keeping
the T12:00:00 UTC-safety convention. Add en/es DAY+MONTH parity cases to
docs/date-label-fixtures.json (TS-only; the Java title formatter stays
German by design) and assert them in the spec.

Refs #666

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/utils/documentDate.spec.ts

@@ -17,9 +17,11 @@ type FixtureCase = {
 	expected: string;
 };
 
+type LocaleFixtureCase = FixtureCase & { locale: string };
+
 const fixtures = JSON.parse(
 	readFileSync(resolve(process.cwd(), '../docs/date-label-fixtures.json'), 'utf-8')
-) as { cases: FixtureCase[] };
+) as { cases: FixtureCase[]; localeCases: LocaleFixtureCase[] };
 
 describe('formatDocumentDate – shared fixture table (de)', () => {
 	for (const c of fixtures.cases) {
@@ -37,6 +39,24 @@ describe('formatDocumentDate – shared fixture table (de)', () => {
 	}
 });
 
+// TS-only locale parity (the Java title formatter is German-only by design, so
+// localeCases are asserted here and never fed to DocumentTitleFormatterTest).
+describe('formatDocumentDate – shared fixture table (en/es locale parity)', () => {
+	for (const c of fixtures.localeCases) {
+		it(`${c.name} [${c.locale}]`, () => {
+			expect(
+				formatDocumentDate(
+					c.anchor,
+					c.precision as Parameters<typeof formatDocumentDate>[1],
+					c.end,
+					c.raw,
+					c.locale
+				)
+			).toBe(c.expected);
+		});
+	}
+});
+
 // ─── Anti-fabrication: suppressed components never leak ──────────────────────
 
 describe('formatDocumentDate – suppressed precision components', () => {
@@ -80,6 +100,40 @@ describe('formatDocumentDate – localization', () => {
 			`${m.date_season_summer(undefined, { locale: 'en' })} 1916`
 		);
 	});
+
+	// DAY precision must honour the active locale (regression: it was hard-wired
+	// to de-DE, so an English/Spanish reader saw "24. Dezember 1943").
+	it('localizes the DAY month name in English', () => {
+		expect(formatDocumentDate('1943-12-24', 'DAY', null, null, 'en')).toBe(
+			new Intl.DateTimeFormat('en', { day: 'numeric', month: 'long', year: 'numeric' }).format(
+				new Date('1943-12-24T12:00:00')
+			)
+		);
+	});
+
+	it('localizes the DAY month name in Spanish', () => {
+		expect(formatDocumentDate('1943-12-24', 'DAY', null, null, 'es')).toBe(
+			new Intl.DateTimeFormat('es', { day: 'numeric', month: 'long', year: 'numeric' }).format(
+				new Date('1943-12-24T12:00:00')
+			)
+		);
+	});
+
+	it('localizes the MONTH month name in English', () => {
+		expect(formatDocumentDate('1916-06-01', 'MONTH', null, 'Juni 1916', 'en')).toBe(
+			new Intl.DateTimeFormat('en', { month: 'long', year: 'numeric' }).format(
+				new Date('1916-06-01T12:00:00')
+			)
+		);
+	});
+
+	it('localizes the MONTH month name in Spanish', () => {
+		expect(formatDocumentDate('1916-06-01', 'MONTH', null, 'Juni 1916', 'es')).toBe(
+			new Intl.DateTimeFormat('es', { month: 'long', year: 'numeric' }).format(
+				new Date('1916-06-01T12:00:00')
+			)
+		);
+	});
 });
 
 // ─── Security: untrusted raw must never influence the structured label ───────

frontend/src/lib/shared/utils/documentDate.ts

@@ -1,4 +1,4 @@
-import { formatDate, formatMCDate } from './date';
+import { formatMCDate } from './date';
 import { m } from '$lib/paraglide/messages.js';
 
 /**
@@ -10,9 +10,11 @@ export type DatePrecision = 'DAY' | 'MONTH' | 'SEASON' | 'YEAR' | 'RANGE' | 'APP
 /**
  * Renders a document date at exactly the precision the data claims — never finer.
  *
- * Delegates to the {@link formatDate}/{@link formatMCDate} helpers (so the
- * `T12:00:00` UTC-safety convention and the German Intl formatting are shared,
- * not reimplemented) and routes every localized word through Paraglide.
+ * Every structured part (month name, day-of-month text, season word, prefixes)
+ * is rendered in the active `locale` — DAY, MONTH and RANGE all go through
+ * `Intl.DateTimeFormat(locale, …)` and the localized words through Paraglide —
+ * so an `en`/`es` reader never sees a German month name. The `T12:00:00`
+ * UTC-safety convention is kept via {@link noon}.
  *
  * The label is the SINGLE SOURCE OF TRUTH shared with the Java
  * {@code DocumentTitleFormatter}: both are asserted against
@@ -42,7 +44,7 @@ export function formatDocumentDate(
 
 	switch (precision) {
 		case 'DAY':
-			return formatDate(iso, 'long');
+			return longDate(iso, locale);
 		case 'MONTH':
 			return monthYear(iso, locale);
 		case 'SEASON':
@@ -60,6 +62,14 @@ export function formatDocumentDate(
 
 // ─── precision branches ──────────────────────────────────────────────────────
 
+function longDate(iso: string, locale: string): string {
+	return new Intl.DateTimeFormat(locale, {
+		day: 'numeric',
+		month: 'long',
+		year: 'numeric'
+	}).format(noon(iso));
+}
+
 function monthYear(iso: string, locale: string): string {
 	return new Intl.DateTimeFormat(locale, { month: 'long', year: 'numeric' }).format(noon(iso));
 }