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/public/sw.js

@@ -27,6 +27,9 @@ self.addEventListener('fetch', (event) => {
     return;
   }
 
+  // Only handle http(s) requests — skip chrome-extension:// etc.
+  if (!url.protocol.startsWith('http')) return;
+
   // Cache-first for static assets
   if (url.pathname.startsWith('/assets/')) {
     event.respondWith(
@@ -50,3 +53,46 @@ self.addEventListener('fetch', (event) => {
   // Default: network with cache fallback
   event.respondWith(fetch(request).catch(() => caches.match(request)));
 });
+
+// --- Push Notifications ---
+
+self.addEventListener('push', (event) => {
+  if (!event.data) return;
+
+  let data;
+  try {
+    data = event.data.json();
+  } catch {
+    // Fallback for plain-text pushes (e.g. browser test pushes)
+    data = { title: 'WealthySmart', body: event.data.text() };
+  }
+
+  const options = {
+    body: data.body,
+    icon: '/icons/icon-192.png',
+    badge: '/icons/icon-192.png',
+    data: { url: data.url || '/' },
+    vibrate: [200, 100, 200],
+    tag: 'transaction',
+    renotify: true,
+  };
+
+  event.waitUntil(self.registration.showNotification(data.title, options));
+});
+
+self.addEventListener('notificationclick', (event) => {
+  event.notification.close();
+
+  const url = event.notification.data?.url || '/';
+  event.waitUntil(
+    clients.matchAll({ type: 'window', includeUncontrolled: true }).then((windowClients) => {
+      for (const client of windowClients) {
+        if (client.url.includes(self.location.origin) && 'focus' in client) {
+          client.navigate(url);
+          return client.focus();
+        }
+      }
+      return clients.openWindow(url);
+    })
+  );
+});