Add images/labels toggle, clean up label styling, merge real racks

- Replace the device-image label background bar and text-shadow with
  NetBox's own technique: a black-stroke pass behind a white-fill pass
  of the same text, giving crisp readable labels over any image
  without a dark box behind them.
- Add an "Images and labels / Images only / Labels only" selector
  above the elevation, mirroring the equivalent control on real NetBox
  rack elevations. The plain label is always in the markup; the
  selector only toggles the image and its over-image label, exactly
  like core NetBox does it. Choice persists per browser via
  localStorage and works without edit permissions.
- Add a "Merge into concept" button to the real dcim Rack list: select
  two racks via the existing bulk-select checkboxes and it forwards
  them to the concept-merge page, pre-filled. New
  deploy.merge_racks_to_concept() combines two real racks' devices
  directly into one new, stacked blueprint without an intermediate
  per-rack copy step.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

netbox_rack_concept/urls.py

@@ -18,6 +18,7 @@ urlpatterns = [
     path('devices/', include(get_model_urls('netbox_rack_concept', 'conceptdevice', detail=False))),
     path('devices/<int:pk>/', include(get_model_urls('netbox_rack_concept', 'conceptdevice'))),
 
-    # Entry point from a real rack
+    # Entry points from real racks
     path('racks/<int:pk>/copy/', views.RackCopyView.as_view(), name='rack_copy'),
+    path('racks/merge/', views.MergeRacksToConceptView.as_view(), name='racks_merge'),
 ]