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/Dashboard.tsx

@@ -9,6 +9,7 @@ import {
   Pencil,
   Check,
   X,
+  BellRing,
 } from 'lucide-react';
 
 import api, { type Account, type Transaction } from '../api';
@@ -124,6 +125,7 @@ export default function Dashboard() {
   const [editValue, setEditValue] = useState('');
   const [exchangeRate, setExchangeRate] = useState<{ buy_rate: number; sell_rate: number } | null>(null);
   const [configSection, setConfigSection] = useState<string | null>(null);
+  const [testingPush, setTestingPush] = useState(false);
 
   const { settings, patchSection } = useSettings();
 
@@ -316,6 +318,48 @@ export default function Dashboard() {
         </CardContent>
       </Card>
 
+      {/* Test push notification */}
+      <Card className="border-dashed border-yellow-500/50">
+        <CardContent className="p-4 flex items-center justify-between">
+          <div>
+            <p className="text-sm font-medium">Test Push Notification</p>
+            <p className="text-xs text-muted-foreground">Creates a mock transaction to trigger a push notification</p>
+          </div>
+          <Button
+            variant="outline"
+            size="sm"
+            disabled={testingPush}
+            onClick={async () => {
+              setTestingPush(true);
+              try {
+                const merchants = ['Walmart', 'AutoMercado', 'Uber Eats', 'Amazon', 'PriceSmart'];
+                const amounts = [4500, 12350, 8900, 25000, 67800];
+                const i = Math.floor(Math.random() * merchants.length);
+                await api.post('/transactions/', {
+                  merchant: merchants[i],
+                  amount: amounts[i],
+                  currency: 'CRC',
+                  date: new Date().toISOString(),
+                  bank: 'BAC',
+                  source: 'CREDIT_CARD',
+                  transaction_type: 'COMPRA',
+                  reference: `test-push-${Date.now()}`,
+                  notes: '[TEST] Push notification test — safe to delete',
+                });
+                fetchData();
+              } catch (e) {
+                console.error('Test push failed:', e);
+              } finally {
+                setTestingPush(false);
+              }
+            }}
+          >
+            <BellRing className="w-4 h-4 mr-2" />
+            {testingPush ? 'Sending...' : 'Send test'}
+          </Button>
+        </CardContent>
+      </Card>
+
       {/* Section config dialog */}
       {configSection && settings.dashboard.sections[configSection] && (
         <SectionConfigDialog