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',

frontend/src/lib/format.ts

@@ -8,6 +8,11 @@ export function formatAmount(amount: number, currency: string) {
   return `₡${abs.toLocaleString('es-CR', { minimumFractionDigits: 2, maximumFractionDigits: 2 })}`;
 }
 
+export function formatLocalDatetime(d: Date): string {
+  const pad = (n: number) => n.toString().padStart(2, '0');
+  return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}T${pad(d.getHours())}:${pad(d.getMinutes())}`;
+}
+
 export function formatDate(dateStr: string) {
   return new Date(dateStr).toLocaleDateString('en-US', { month: 'short', day: 'numeric' });
 }

frontend/src/pages/Dashboard.tsx

@@ -15,7 +15,7 @@ import {
 
 import api, { type Account, type Transaction } from '../api';
 import { useSettings } from '@/hooks/useSettings';
-import { formatAmount, formatDate } from '@/lib/format';
+import { formatAmount, formatDate, formatLocalDatetime } from '@/lib/format';
 import DashboardSection from '@/components/DashboardSection';
 import SectionConfigDialog from '@/components/SectionConfigDialog';
 import { Button } from '@/components/ui/button';
@@ -340,7 +340,7 @@ export default function Dashboard() {
                   merchant: merchants[i],
                   amount: amounts[i],
                   currency: 'CRC',
-                  date: new Date().toISOString(),
+                  date: formatLocalDatetime(new Date()),
                   bank: 'BAC',
                   source: 'CREDIT_CARD',
                   transaction_type: 'COMPRA',