feat: enable color selection for cart items and enhance UI integration

- Added `selected_color` field to `CartItem` model with migration
- Updated color swatch behavior on product detail page with selection logic and disabled button handling
- Improved cart templates to display selected color alongside item details
- Modified `add_item_to_cart` service and `add` view to handle color selection
- Enhanced styles for active swatches and cart item color display

orders/migrations/0003_cartitem_selected_color.py

@@ -0,0 +1,20 @@
+# Generated by Django 6.0.5 on 2026-05-25 22:37
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('items', '0005_remove_item_color_item_colors'),
+        ('orders', '0002_order_orderitem'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='cartitem',
+            name='selected_color',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, to='items.color', verbose_name='Выбранный цвет'),
+        ),
+    ]