Fix rear-face image-only labels, click-to-add units, cable concept devices

- Fix "images only" leaving plain labels visible on faces where a
  device has no image for that side (e.g. no rear_image set): the
  view selector now hides every plain label outright in that mode,
  not just the ones an image happens to sit on top of.
- Remove the "Add planned device" button from the elevation card.
  Empty units are now click targets themselves - clicking one opens
  the add-device form pre-filled with that position and face, exactly
  like a real rack's elevation. Occupancy is computed per unit per
  face from the existing device blocks.
- Add cabling for concept devices: ConceptComponent (interfaces,
  front/rear ports, power ports/outlets, console ports) and
  ConceptCable (a connection between two components, restricted to
  devices in the same concept). components.sync_components()
  populates a device's components from its device type's templates,
  including the front/rear port pass-through mapping copied from
  DeviceType.port_mappings - so a device with front/rear port
  templates (a patch panel being the common case) behaves exactly
  like a real one, without hardcoding anything role-specific.
  Placeholder devices without a device type get components added by
  hand. Full CRUD views, tables, filtersets, API endpoints and a
  "Connect" action wired in from the device detail page.

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

netbox_rack_concept/navigation.py

@@ -44,6 +44,16 @@ device_buttons = [
     ),
 ]
 
+cable_buttons = [
+    PluginMenuButton(
+        link='plugins:netbox_rack_concept:conceptcable_add',
+        title=_('Add'),
+        icon_class='mdi mdi-plus-thick',
+        color=ButtonColorChoices.GREEN,
+        permissions=['netbox_rack_concept.add_conceptcable']
+    ),
+]
+
 menu = PluginMenu(
     label=_('Rack Concepts'),
     icon_class='mdi mdi-view-grid-plus-outline',
@@ -62,5 +72,18 @@ menu = PluginMenu(
                 buttons=device_buttons
             ),
         )),
+        (_('Cabling'), (
+            PluginMenuItem(
+                link='plugins:netbox_rack_concept:conceptcomponent_list',
+                link_text=_('Components'),
+                permissions=['netbox_rack_concept.view_conceptcomponent'],
+            ),
+            PluginMenuItem(
+                link='plugins:netbox_rack_concept:conceptcable_list',
+                link_text=_('Cables'),
+                permissions=['netbox_rack_concept.view_conceptcable'],
+                buttons=cable_buttons
+            ),
+        )),
     )
 )