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/services/inventory_engine.py

@@ -120,11 +120,32 @@ def record_sale(conn, inventory_id, branch_id, quantity, sale_id=None, cost_at_t
     NOT exposed via HTTP endpoint — called directly by the POS blueprint (Plan 3)
     which imports inventory_engine as part of the full sale transaction.
     """
-    return record_operation(
+    op_id = record_operation(
         conn, inventory_id, branch_id, 'SALE', -abs(quantity),
         reference_id=sale_id, reference_type='sale', cost_at_time=cost_at_time
     )
 
+    # Check if stock hit zero — push to owner (best-effort)
+    try:
+        remaining = get_stock(conn, inventory_id, branch_id)
+        if remaining <= 0:
+            cur = conn.cursor()
+            cur.execute("SELECT part_number, name FROM inventory WHERE id = %s", (inventory_id,))
+            inv_row = cur.fetchone()
+            cur.close()
+            if inv_row:
+                from services.push_service import notify_owner
+                notify_owner(
+                    conn,
+                    'Stock en Cero',
+                    f'{inv_row[1] or inv_row[0]} se quedo sin existencias',
+                    '/pos'
+                )
+    except Exception:
+        pass  # Push failures never block sales
+
+    return op_id
+
 
 def record_return(conn, inventory_id, branch_id, quantity, sale_id=None, notes=None):
     """Record a customer return (positive quantity)."""