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/template_content.py

@@ -10,7 +10,10 @@ def _enabled():
 
 
 class RackConceptButtons(PluginTemplateExtension):
-    """Adds "Copy to concept" to the button row of a rack, and lists derived concepts."""
+    """
+    Adds "Copy to concept" to the button row of a rack, "Merge into concept" to the
+    rack list's toolbar, and lists concepts derived from a rack.
+    """
     models = ['dcim.rack']
 
     def buttons(self):
@@ -22,6 +25,13 @@ class RackConceptButtons(PluginTemplateExtension):
             'rack': self.context['object'],
         })
 
+    def list_buttons(self):
+        if not _enabled():
+            return ''
+        if not self.context['request'].user.has_perm('netbox_rack_concept.add_rackconcept'):
+            return ''
+        return self.render('netbox_rack_concept/inc/rack_list_merge_button.html')
+
     def right_page(self):
         if not _enabled():
             return ''