Fix QZ Tray library loading and add pairing keys API endpoint

- Fix QZ Tray library loading errors (net::ERR_NAME_NOT_RESOLVED) by switching from CDN to local static file
- Add /labels/api/pairing-keys endpoint to fetch valid printer pairing keys
- Update print_labels.html to use local js/qz-tray.js
- Update print_lost_labels.html to use local js/qz-tray.js
- Update print_lost_labels_new.html to use correct path js/qz-tray.js
- Update fg_scan.html to use local js/qz-tray.js
- API returns active pairing keys from qz_pairing_keys table for printer selection

app/modules/labels/routes.py

@@ -487,4 +487,39 @@ def api_generate_batch_pdf(order_id):
         
     except Exception as e:
         logger.error(f"Error generating batch PDF: {e}")
+        return jsonify({'error': str(e)}), 500
+
+
+@labels_bp.route('/api/pairing-keys', methods=['GET'], endpoint='api_pairing_keys')
+def api_pairing_keys():
+    """Get QZ Tray pairing keys for printer selection"""
+    if 'user_id' not in session:
+        return jsonify({'error': 'Unauthorized'}), 401
+    
+    try:
+        conn = get_db()
+        cursor = conn.cursor()
+        
+        cursor.execute("""
+            SELECT id, printer_name, pairing_key, valid_until 
+            FROM qz_pairing_keys 
+            WHERE valid_until >= CURDATE()
+            ORDER BY printer_name ASC
+        """)
+        
+        pairing_keys = []
+        for row in cursor.fetchall():
+            pairing_keys.append({
+                'id': row[0],
+                'printer_name': row[1],
+                'pairing_key': row[2]
+            })
+        
+        cursor.close()
+        
+        logger.info(f"Retrieved {len(pairing_keys)} valid pairing keys")
+        return jsonify(pairing_keys), 200
+        
+    except Exception as e:
+        logger.error(f"Error fetching pairing keys: {e}")
         return jsonify({'error': str(e)}), 500