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/blueprints/cashregister_bp.py

@@ -345,6 +345,21 @@ def cut_z():
                    })
 
         conn.commit()
+
+        # Push notification to owner if cash difference > $500
+        if abs(difference) > 500:
+            try:
+                from services.push_service import notify_owner
+                emp_name = getattr(g, 'employee_name', 'Empleado')
+                notify_owner(
+                    conn,
+                    'Diferencia en Caja',
+                    f'Corte Z caja #{register_id}: diferencia de ${difference:,.2f} ({emp_name})',
+                    '/pos'
+                )
+            except Exception:
+                pass  # Push failures never block business logic
+
         cur.close(); conn.close()
 
         return jsonify({