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/_fragments/item-grid.html

@@ -0,0 +1,7 @@
+<div class="items-grid"{% if favorites_grid %} data-favorites-grid data-empty-message="{{ empty_message|escape }}"{% endif %}>
+    {% for item in items %}
+        {% include "catalog/_fragments/item-card.html" with item=item %}
+    {% empty %}
+        <p class="item-list-empty">{{ empty_message }}</p>
+    {% endfor %}
+</div>