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/static/netbox_rack_concept/elevation.css

@@ -22,6 +22,27 @@
     fill: var(--tblr-primary-lt, #d7e7fb);
 }
 
+.concept-elevation .unit-slot.free {
+    cursor: pointer;
+}
+
+.concept-elevation .unit-slot.free:hover {
+    fill: var(--tblr-primary-lt, #d7e7fb);
+}
+
+/* "+ Add device" hint, shown only while hovering an empty unit - same trick as
+   NetBox's own rack elevation. */
+.concept-elevation .add-device-hint {
+    font-size: 10px;
+    fill: var(--tblr-primary, #206bc4);
+    opacity: 0;
+    pointer-events: none;
+}
+
+.concept-elevation a:hover .add-device-hint {
+    opacity: 1;
+}
+
 .concept-elevation .unit-label {
     font-size: 10px;
     fill: var(--tblr-secondary, #808f9c);

netbox_rack_concept/static/netbox_rack_concept/elevation.js

@@ -18,12 +18,21 @@
     function applyView(view) {
         const showImages = view !== 'labels-only';
         const showImageLabels = view === 'images-and-labels';
+        // "Images only" hides every plain label outright, not just the ones sitting
+        // under an image: a device without an image for this face has nothing to
+        // visually cover its label otherwise, which would leave text showing (e.g. on
+        // a rear face where only the front image is set) even though the user picked
+        // "images only".
+        const hidePlainLabels = view === 'images-only';
         container.querySelectorAll('image.device-image').forEach(function (el) {
             el.classList.toggle('hidden', !showImages);
         });
         container.querySelectorAll('.device-image-label').forEach(function (el) {
             el.classList.toggle('hidden', !showImageLabels);
         });
+        container.querySelectorAll('.device-label').forEach(function (el) {
+            el.classList.toggle('hidden', hidePlainLabels);
+        });
     }
 
     let initial = 'images-and-labels';