feat: Add SKU field to ProductKit form for kit creation

- Added 'sku' field to ProductKitForm meta fields list
- Added SKU label in form labels
- Added SKU widget styling in __init__ method with helpful placeholder
- Updated productkit_form.html template to display SKU field after name, before description
- Updated form field filtering to exclude 'sku' from dynamic loop to prevent duplication

🤖 Generated with Claude Code

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

myproject/products/templates/products/productkit_form.html

@@ -140,6 +140,18 @@
                             {% endif %}
                         </div>
 
+                        <!-- Артикул -->
+                        <div class="mb-3">
+                            {{ form.sku.label_tag }}
+                            {{ form.sku }}
+                            {% if form.sku.help_text %}
+                                <small class="form-text text-muted">{{ form.sku.help_text }}</small>
+                            {% endif %}
+                            {% if form.sku.errors %}
+                                <div class="text-danger">{{ form.sku.errors }}</div>
+                            {% endif %}
+                        </div>
+
                         <!-- Категория -->
                         <div class="mb-3">
                             <div class="d-flex justify-content-between align-items-center mb-2">
@@ -175,7 +187,7 @@
                         </div>
 
                         {% for field in form %}
-                            {% if field.name not in 'name,categories,description,tags' %}
+                            {% if field.name not in 'name,sku,categories,description,tags' %}
                             {% if field.name == 'fixed_price' %}
                             <!-- Alert для неактивного поля fixed_price -->
                             <div id="fixed-price-alert" class="alert alert-warning d-none mb-3" role="alert">