feat: implement shopping cart feature with user authentication support

- Added `Cart` and `CartItem` models with migrations for managing user-specific carts and items
- Developed services for cart-related operations: adding, removing, and updating cart items
- Built specialized templates (`cart.html`) and styled them with new CSS assets (`cart.css`)
- Enabled real-time UI updates via AJAX in shopping cart interactions (`cart.js`)
- Modified header to dynamically display cart link with user authentication awareness
- Added Leaflet-based interactive map to the contacts page and integrated CSS/JS dependencies
- Expanded tests for cart functionality, item detail page cart integration, and contacts page maps

templates/_layouts/base.html

@@ -54,5 +54,7 @@
     })
 </script>
 <script src="{% static 'js/favorites.js' %}"></script>
+<script src="{% static 'js/cart.js' %}"></script>
+{% block add_js %}{% endblock %}
 </body>
 </html>