Add budget module and push notifications for transactions

Budget: recurring items CRUD, yearly/monthly projections with no-double-count
logic, and full UI (overview, monthly detail, recurring items manager).

Push notifications: Web Push via VAPID keys, triggered on transaction creation
from n8n. Includes service worker handlers, frontend subscription flow, and
a test button on the Dashboard (temporary).

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

frontend/src/components/Layout.tsx

@@ -1,18 +1,18 @@
 import { NavLink, Outlet, useNavigate } from 'react-router-dom';
 import {
   LayoutDashboard,
-  CreditCard,
+  Calculator,
   BarChart3,
-  ArrowLeftRight,
   LogOut,
   Wallet,
   Menu,
   Sun,
   Moon,
 } from 'lucide-react';
-import { useState } from 'react';
+import { useEffect, useState } from 'react';
 import { useAuth } from '../AuthContext';
 import { useTheme } from '../ThemeContext';
+import { subscribeToPush } from '../pushNotifications';
 import { Button } from '@/components/ui/button';
 import {
   Sheet,
@@ -26,9 +26,8 @@ import { cn } from '@/lib/utils';
 
 const navItems = [
   { to: '/', icon: LayoutDashboard, label: 'Dashboard' },
-  { to: '/transactions', icon: CreditCard, label: 'Transactions' },
+  { to: '/budget', icon: Calculator, label: 'Presupuesto' },
   { to: '/analytics', icon: BarChart3, label: 'Analytics' },
-  { to: '/transfers', icon: ArrowLeftRight, label: 'Cash & Transfers' },
 ];
 
 export default function Layout() {
@@ -37,6 +36,10 @@ export default function Layout() {
   const navigate = useNavigate();
   const [mobileOpen, setMobileOpen] = useState(false);
 
+  useEffect(() => {
+    subscribeToPush();
+  }, []);
+
   const handleLogout = () => {
     logout();
     navigate('/login');