feat(pos): add 3 improvements — Spanish translations, PDF quotes, push notifications

1. Spanish translations for TecDoc catalog (translations.py) applied to
   catalog_service.py and dashboard server.py endpoints
2. Printable quotation HTML endpoint (/pos/api/quotations/<id>/pdf) with
   @media print CSS for clean browser-to-PDF output
3. Web Push notifications to owner/admin on sale cancellation, stock zero,
   and cash register differences > $500. Includes service worker, VAPID
   key management, and subscription endpoints.

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

pos/static/sw-push.js

@@ -0,0 +1,55 @@
+/**
+ * sw-push.js — Service Worker for Nexus POS push notifications
+ */
+
+self.addEventListener('push', function(event) {
+    var data = { title: 'Nexus POS', body: '', url: '/pos', icon: '/pos/static/icons/icon-192.png' };
+
+    if (event.data) {
+        try {
+            data = Object.assign(data, event.data.json());
+        } catch(e) {
+            data.body = event.data.text();
+        }
+    }
+
+    var options = {
+        body: data.body,
+        icon: data.icon || '/pos/static/icons/icon-192.png',
+        badge: data.badge || '/pos/static/icons/badge-72.png',
+        vibrate: [200, 100, 200],
+        data: { url: data.url || '/pos' },
+        actions: [
+            { action: 'open', title: 'Ver' },
+            { action: 'dismiss', title: 'Cerrar' }
+        ]
+    };
+
+    event.waitUntil(
+        self.registration.showNotification(data.title, options)
+    );
+});
+
+self.addEventListener('notificationclick', function(event) {
+    event.notification.close();
+
+    if (event.action === 'dismiss') return;
+
+    var url = (event.notification.data && event.notification.data.url) || '/pos';
+
+    event.waitUntil(
+        clients.matchAll({ type: 'window', includeUncontrolled: true }).then(function(clientList) {
+            // Focus existing window if open
+            for (var i = 0; i < clientList.length; i++) {
+                var client = clientList[i];
+                if (client.url.indexOf('/pos') !== -1 && 'focus' in client) {
+                    return client.focus();
+                }
+            }
+            // Open new window
+            if (clients.openWindow) {
+                return clients.openWindow(url);
+            }
+        })
+    );
+});