Add full CRUD interface for Showcase management

Implemented complete web interface for managing showcases (display areas for ready-made bouquets) with:

**Backend:**
- ShowcaseForm with validation (unique name per warehouse)
- ShowcaseListView with filtering by warehouse and status
- ShowcaseCreateView, ShowcaseUpdateView with success messages
- ShowcaseDeleteView with active reservation validation
- URL routes: list, create, edit, delete

**Frontend:**
- List page with warehouse grouping, active reservations count
- Responsive table with filters (warehouse, status)
- Create/edit form with Bootstrap styling
- Delete confirmation with active reservations check
- Breadcrumb navigation

**Features:**
✅ One warehouse can have multiple showcases (ForeignKey relationship)
✅ Unique showcase names within each warehouse
✅ Display active reservation counts for each showcase
✅ Prevent deletion if showcase has active reservations
✅ Auto-select default warehouse when creating showcase
✅ Navigation link added to main navbar between "Касса" and "Склад"
✅ Active state highlighting in navigation

**Files created:**
- inventory/forms_showcase.py (ShowcaseForm)
- inventory/views/showcase.py (4 CBV views)
- inventory/templates/inventory/showcase/ (list, form, delete templates)

**Files modified:**
- inventory/urls.py (added showcase routes)
- inventory/forms.py (added Showcase import)
- templates/navbar.html (added "Витрины" link)

URL structure:
/inventory/showcases/ - list
/inventory/showcases/create/ - create
/inventory/showcases/<id>/edit/ - edit
/inventory/showcases/<id>/delete/ - delete

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

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

myproject/inventory/forms.py

@@ -5,7 +5,7 @@ from decimal import Decimal
 
 from .models import (
     Warehouse, Incoming, Sale, WriteOff, Transfer, Reservation, Inventory, InventoryLine, StockBatch,
-    TransferBatch, TransferItem
+    TransferBatch, TransferItem, Showcase
 )
 from products.models import Product
 
@@ -460,3 +460,4 @@ class TransferBulkForm(forms.Form):
                 raise ValidationError('Склад-источник и склад-назначение должны быть разными')
 
         return cleaned_data
+