refactor(frontend): extract toActionResult helper and formatDate utility

- Admin page: replace 7 identical error-handling blocks with a single
  toActionResult() helper — DRY without over-abstraction
- New date.ts util: formatDate(isoDate) centralises the T12:00:00
  timezone guard and Intl.DateTimeFormat locale config

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/utils/date.ts

@@ -0,0 +1,11 @@
+/**
+ * Format an ISO date string (YYYY-MM-DD) for display.
+ * Uses T12:00:00 to avoid UTC timezone off-by-one when converting to local time.
+ */
+export function formatDate(isoDate: string): string {
+    return new Intl.DateTimeFormat('de-DE', {
+        day: 'numeric',
+        month: 'long',
+        year: 'numeric'
+    }).format(new Date(isoDate + 'T12:00:00'));
+}