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

static/js/cart.js

@@ -0,0 +1,20 @@
+document.addEventListener('DOMContentLoaded', function () {
+    document.querySelectorAll('[data-cart-decrement]').forEach(function (button) {
+        button.addEventListener('click', function () {
+            const quantity = Number(button.dataset.currentQuantity || '0');
+            const removeForm = document.getElementById(button.dataset.removeFormId);
+            const updateForm = document.getElementById(button.dataset.updateFormId);
+
+            if (quantity <= 1) {
+                if (window.confirm('Удалить товар из корзины?') && removeForm) {
+                    removeForm.submit();
+                }
+                return;
+            }
+
+            if (updateForm) {
+                updateForm.submit();
+            }
+        });
+    });
+});