Phase 0.3: migrate date formatting to date-fns

frontend/src/components/mail/MailList.tsx

@@ -11,6 +11,7 @@ import { decodeMimeHeader } from '@/api/mail';
 import { EmptyState } from '@/components/ui/EmptyState';
 import { Pagination } from '@/components/ui/Pagination';
 import { ChevronUp, Loader2, Paperclip, Star } from 'lucide-react';
+import { formatSmartDate } from '@/utils/date';
 
 export interface MailListProps {
   mails: Mail[];
@@ -30,12 +31,7 @@ export interface MailListProps {
 }
 
 function formatDate(dateStr: string): string {
-  const date = new Date(dateStr);
- const now = new Date();
-  if (date.toDateString() === now.toDateString()) {
-    return date.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' });
-  }
-  return date.toLocaleDateString([], { month: 'short', day: 'numeric' });
+  return formatSmartDate(dateStr) || dateStr;
 }
 
 export function MailList({