Add default showcase selection per warehouse

- Add is_default field to Showcase model with unique constraint per warehouse
- Implement Showcase.save() to ensure only one default per warehouse
- Add SetDefaultShowcaseView for AJAX-based default selection
- Update ShowcaseForm to include is_default checkbox
- Add interactive checkbox UI in showcase list with AJAX functionality
- Update POS API to return showcase.is_default instead of warehouse.is_default
- Update terminal.js to auto-select showcase based on its is_default flag
- Add migration for is_default field

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

myproject/inventory/migrations/0004_showcase_is_default_and_more.py

@@ -0,0 +1,22 @@
+# Generated by Django 5.0.10 on 2025-11-20 08:08
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('inventory', '0003_showcase_reservation_showcase_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='showcase',
+            name='is_default',
+            field=models.BooleanField(default=False, verbose_name='По умолчанию'),
+        ),
+        migrations.AddIndex(
+            model_name='showcase',
+            index=models.Index(fields=['is_default'], name='inventory_s_is_defa_bf9a7c_idx'),
+        ),
+    ]