Fix date picker defaulting to UTC instead of browser local time

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/TransactionModal.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useState } from 'react';
 import api, { type Category, type Transaction } from '../api';
+import { formatLocalDatetime } from '@/lib/format';
 import { Button } from '@/components/ui/button';
 import { Input } from '@/components/ui/input';
 import { Label } from '@/components/ui/label';
@@ -33,7 +34,7 @@ export default function TransactionModal({ transaction, source, onClose, onSaved
     merchant: '',
     amount: '',
     currency: 'CRC',
-    date: new Date().toISOString().slice(0, 16),
+    date: formatLocalDatetime(new Date()),
     transaction_type: 'COMPRA',
     source,
     bank: 'BAC',