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/_fragments/header.html

@@ -85,7 +85,7 @@
                         <path d="m8 2.748-.717-.737C5.6.281 2.514.878 1.4 3.053c-.523 1.023-.641 2.5.314 4.385.92 1.815 2.834 3.989 6.286 6.357 3.452-2.368 5.365-4.542 6.286-6.357.955-1.886.838-3.362.314-4.385C13.486.878 10.4.28 8.717 2.01zM8 15C-7.333 4.868 3.279-3.04 7.824 1.143q.09.083.176.171a3 3 0 0 1 .176-.17C12.72-3.042 23.333 4.867 8 15"></path>
                     </svg>
                 </a>
-                <a href="#">
+                <a href="{% if request.user.is_authenticated %}{% url 'orders:cart' %}{% else %}{% url 'users:login' %}?next={% url 'orders:cart' %}{% endif %}" aria-label="Корзина">
                     <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor"
                          class="bi bi-cart2" viewBox="0 0 16 16">
                         <path d="M0 2.5A.5.5 0 0 1 .5 2H2a.5.5 0 0 1 .485.379L2.89 4H14.5a.5.5 0 0 1 .485.621l-1.5 6A.5.5 0 0 1 13 11H4a.5.5 0 0 1-.485-.379L1.61 3H.5a.5.5 0 0 1-.5-.5M3.14 5l1.25 5h8.22l1.25-5zM5 13a1 1 0 1 0 0 2 1 1 0 0 0 0-2m-2 1a2 2 0 1 1 4 0 2 2 0 0 1-4 0m9-1a1 1 0 1 0 0 2 1 1 0 0 0 0-2m-2 1a2 2 0 1 1 4 0 2 2 0 0 1-4 0"></path>