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/js/push.js

@@ -0,0 +1,95 @@
+/**
+ * push.js — Web Push notification setup for Nexus POS
+ *
+ * Registers a service worker and subscribes to push notifications.
+ * Only activates for owner/admin roles.
+ */
+(function() {
+    'use strict';
+
+    // Only set up push for owner/admin
+    var employee = {};
+    try { employee = JSON.parse(localStorage.getItem('pos_employee') || '{}'); } catch(e) {}
+    var role = employee.role || '';
+    if (role !== 'owner' && role !== 'admin') return;
+
+    // Check browser support
+    if (!('serviceWorker' in navigator) || !('PushManager' in window)) {
+        console.log('[Push] Browser does not support push notifications');
+        return;
+    }
+
+    var token = localStorage.getItem('pos_token');
+    if (!token) return;
+
+    function urlBase64ToUint8Array(base64String) {
+        var padding = '='.repeat((4 - base64String.length % 4) % 4);
+        var base64 = (base64String + padding).replace(/-/g, '+').replace(/_/g, '/');
+        var rawData = window.atob(base64);
+        var outputArray = new Uint8Array(rawData.length);
+        for (var i = 0; i < rawData.length; ++i) {
+            outputArray[i] = rawData.charCodeAt(i);
+        }
+        return outputArray;
+    }
+
+    async function setupPush() {
+        try {
+            // Register service worker
+            var registration = await navigator.serviceWorker.register('/pos/static/sw-push.js', {
+                scope: '/pos/'
+            });
+            console.log('[Push] Service worker registered');
+
+            // Get VAPID key from server
+            var resp = await fetch('/pos/api/push/vapid-key', {
+                headers: { 'Authorization': 'Bearer ' + token }
+            });
+            if (!resp.ok) {
+                console.log('[Push] VAPID key not available:', resp.status);
+                return;
+            }
+            var data = await resp.json();
+            var vapidKey = data.public_key;
+            if (!vapidKey) return;
+
+            // Request permission
+            var permission = await Notification.requestPermission();
+            if (permission !== 'granted') {
+                console.log('[Push] Permission denied');
+                return;
+            }
+
+            // Subscribe
+            var subscription = await registration.pushManager.subscribe({
+                userVisibleOnly: true,
+                applicationServerKey: urlBase64ToUint8Array(vapidKey)
+            });
+
+            // Send subscription to server
+            var subResp = await fetch('/pos/api/push/subscribe', {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json',
+                    'Authorization': 'Bearer ' + token
+                },
+                body: JSON.stringify({ subscription: subscription.toJSON() })
+            });
+
+            if (subResp.ok) {
+                console.log('[Push] Subscribed successfully');
+            }
+        } catch(err) {
+            console.log('[Push] Setup error:', err);
+        }
+    }
+
+    // Delay push setup to not block page load
+    if (document.readyState === 'complete') {
+        setTimeout(setupPush, 2000);
+    } else {
+        window.addEventListener('load', function() {
+            setTimeout(setupPush, 2000);
+        });
+    }
+})();