feat(pos): add editable showcase creation date for kits

- Add showcase_created_at field to ProductKit model
- Display days ago as badge in product card (0 дней, 1 день, etc.)
- Add date input field in edit modal
- Auto-set current date/time for new showcase kits

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

myproject/products/migrations/0003_productkit_showcase_created_at.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.10 on 2026-01-23 22:05
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('products', '0002_bouquetname'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='productkit',
+            name='showcase_created_at',
+            field=models.DateTimeField(blank=True, help_text='Дата создания букета для витрины (редактируемая)', null=True, verbose_name='Дата размещения на витрине'),
+        ),
+    ]