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/templates/pos.html

@@ -1478,6 +1478,7 @@
        JAVASCRIPT
        ================================================================ -->
   <script src="/pos/static/js/app-init.js"></script>
+  <script src="/pos/static/js/push.js"></script>
   <script src="/pos/static/js/pos.js"></script>
 
   <script>