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/pages/Login.tsx

@@ -4,6 +4,7 @@ import { Wallet, ArrowRight, AlertCircle } from 'lucide-react';
 
 import { login } from '../api';
 import { useAuth } from '../AuthContext';
+import { subscribeToPush } from '../pushNotifications';
 import { Button } from '@/components/ui/button';
 import { Input } from '@/components/ui/input';
 import { Label } from '@/components/ui/label';
@@ -24,6 +25,7 @@ export default function Login() {
     try {
       await login(username, password);
       setAuthenticated(true);
+      subscribeToPush();
       navigate('/');
     } catch {
       setError('Invalid credentials');