fix(relativeTime): guard against Invalid Date producing NaN strings

If a row ever receives a malformed uploadedAt (e.g. manual SQL migration,
backend regression), the helper now falls back to "vor 0 Minute(n)"
rather than rendering "vor NaN Tag(en)" to the user.

Addresses Nora's review suggestion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/relativeTime.spec.ts

@@ -31,4 +31,11 @@ describe('relativeTimeDe', () => {
 		expect(relativeTimeDe(NOW, NOW)).toMatch(/0/);
 		expect(relativeTimeDe(NOW, NOW)).toMatch(/Minute/i);
 	});
+
+	it('falls back to 0 minutes when the input Date is invalid', () => {
+		const invalid = new Date('not-a-real-date');
+		// Never crash the UI if the backend ever ships a malformed uploadedAt.
+		expect(relativeTimeDe(invalid, NOW)).toMatch(/0/);
+		expect(relativeTimeDe(invalid, NOW)).toMatch(/Minute/i);
+	});
 });

frontend/src/lib/relativeTime.ts

@@ -2,6 +2,9 @@ import * as m from '$lib/paraglide/messages.js';
 
 export function relativeTimeDe(from: Date, now: Date = new Date()): string {
 	const minutes = Math.round((now.getTime() - from.getTime()) / 60_000);
+	// Malformed input (e.g. Invalid Date from a broken backend string) must not
+	// crash the dashboard — fall back to "0 Minuten" rather than render NaN.
+	if (!Number.isFinite(minutes)) return m.comment_time_minutes({ count: 0 });
 	if (minutes < 60) return m.comment_time_minutes({ count: minutes });
 	if (minutes < 1440) return m.comment_time_hours({ count: Math.round(minutes / 60) });
 	return m.comment_time_days({ count: Math.round(minutes / 1440) });