Phase 0.3: migrate date formatting to date-fns

frontend/src/components/mail/MailDetail.tsx

@@ -11,6 +11,7 @@ import { decodeMimeHeader } from '@/api/mail';
 import { Button } from '@/components/ui/Button';
 import { EmptyState } from '@/components/ui/EmptyState';
 import { FileText, Loader2 } from 'lucide-react';
+import { formatDateTime } from '@/utils/date';
 
 export interface MailDetailProps {
   mail: Mail | null; loading: boolean;
@@ -26,14 +27,7 @@ export interface MailDetailProps {
 }
 
 function formatFullDate(dateStr: string): string {
-  const date = new Date(dateStr);
-  return date.toLocaleString([], {
-    year: 'numeric',
-    month: 'short',
-    day: 'numeric',
-    hour: '2-digit',
-    minute: '2-digit',
-  });
+  return formatDateTime(dateStr) || dateStr;
 }
 
 function formatBytes(bytes: number): string {