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/templates/inventory/showcase/form.html

@@ -85,7 +85,7 @@
             </div>
 
             <!-- Is Active -->
-            <div class="mb-4">
+            <div class="mb-3">
               <div class="form-check">
                 {{ form.is_active }}
                 <label class="form-check-label" for="{{ form.is_active.id_for_label }}">
@@ -102,6 +102,24 @@
               {% endif %}
             </div>
 
+            <!-- Is Default -->
+            <div class="mb-4">
+              <div class="form-check">
+                {{ form.is_default }}
+                <label class="form-check-label" for="{{ form.is_default.id_for_label }}">
+                  {{ form.is_default.label }}
+                </label>
+                {% if form.is_default.help_text %}
+                  <div><small class="form-text text-muted">{{ form.is_default.help_text }}</small></div>
+                {% endif %}
+              </div>
+              {% if form.is_default.errors %}
+                <div class="invalid-feedback d-block">
+                  {{ form.is_default.errors }}
+                </div>
+              {% endif %}
+            </div>
+
             <!-- Buttons -->
             <div class="d-flex gap-2">
               <button type="submit" class="btn btn-primary">