feat: add favorites feature for users

- Implemented `Favorite` model with unique user-item constraint and timestamped records
- Added `favorites:index` and `favorites:toggle` views with AJAX support for toggling favorites
- Built templates and front-end assets for the favorites page and integration with item grids
- Extended header with favorites link and dynamic counter
- Added tests covering models, views, and templates for the favorites functionality
- Updated admin to enable management of favorites via Django admin interface

templates/catalog/_fragments/item-list.html

@@ -1,34 +1,3 @@
 <main class="item-list">
-    <div class="items-grid">
-    {% for item in items %}
-        <article class="item-card">
-            <a class="item-card-link" href="{% url 'detail' item.category.slug item.slug %}" aria-label="Открыть товар {{ item.name }}">
-                {% with primary_image=item.primary_image %}
-                    <div class="item-card-image">
-                        {% if primary_image %}
-                            <img src="{{ primary_image.image.url }}" alt="{{ primary_image.alt|default:item.name }}">
-                        {% else %}
-                            <div class="item-card-placeholder">Нет фото</div>
-                        {% endif %}
-                    </div>
-                {% endwith %}
-
-                <div class="item-card-body">
-                    <div class="item-card-category">
-                        {{ item.category.name }}{% if item.subcategory %} / {{ item.subcategory.name }}{% endif %}
-                    </div>
-                    <h2 class="item-card-title">{{ item.name }}</h2>
-                    <div class="item-card-footer">
-                        <span class="item-card-price">{{ item.price }} руб.</span>
-                        <span class="item-card-availability {% if item.is_available %}available{% else %}unavailable{% endif %}">
-                            {% if item.is_available %}В наличии{% else %}Нет в наличии{% endif %}
-                        </span>
-                    </div>
-                </div>
-            </a>
-        </article>
-    {% empty %}
-        <p class="item-list-empty">В этой категории пока нет товаров.</p>
-    {% endfor %}
-    </div>
+    {% include "_fragments/item-grid.html" with items=items empty_message="В этой категории пока нет товаров." %}
 </main>