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/catalog/detail.html

@@ -61,7 +61,28 @@
             {% endif %}
 
             <div class="product-summary__actions">
-                <button class="product-summary__button product-summary__button--primary" type="button">В корзину</button>
+                {% if item.id in cart_item_ids %}
+                    <a class="product-summary__button product-summary__button--primary product-summary__button--link" href="{% url 'orders:cart' %}">
+                        Перейти в корзину
+                    </a>
+                {% elif request.user.is_authenticated %}
+                    <form method="post" action="{% url 'orders:add' %}" class="product-summary__cart-form">
+                        {% csrf_token %}
+                        <input type="hidden" name="item_id" value="{{ item.id }}">
+                        <input type="hidden" name="next" value="{{ request.get_full_path }}">
+                        <button class="product-summary__button product-summary__button--primary" type="submit" {% if not item.is_available %}disabled{% endif %}>
+                            В корзину
+                        </button>
+                    </form>
+                {% else %}
+                    <a
+                        class="product-summary__button product-summary__button--primary product-summary__button--link"
+                        href="{% url 'users:login' %}?next={{ request.get_full_path|urlencode }}"
+                        {% if not item.is_available %}aria-disabled="true"{% endif %}
+                    >
+                        В корзину
+                    </a>
+                {% endif %}
                 <button class="product-summary__button product-summary__button--secondary" type="button">Купить</button>
             </div>
         </section>