Fix elevation overlap, add device images, rack merge, and size override

- Fix elevation rendering bug: device blocks were drawn MARGIN px
  further right than the rack frame/unit slots, causing them to
  overlap and extend past the right border. All three now share one
  RACK_X origin.
- Show the device type's front/rear image inside each elevation block
  (clipped to the block, translucent label bar on top for legibility)
  when one is set on the device type; falls back to a plain colour
  block with text otherwise.
- Add "Merge" action to combine two concepts into one taller one (e.g.
  two half-height racks stacked into a full-height rack): the top
  concept's devices are shifted up by the bottom concept's height,
  neither source is modified. Available from the plugin menu, from a
  concept's detail page, and via POST /concepts/merge/.
- Copying a real rack into a concept can now target a different width,
  height and/or starting unit than the source rack; devices that no
  longer fit at the chosen size are skipped and reported as warnings
  instead of aborting the copy.

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

netbox_rack_concept/static/netbox_rack_concept/elevation.css

@@ -37,11 +37,25 @@
     stroke-width: 1;
 }
 
+.concept-elevation .device-image {
+    pointer-events: none;
+}
+
+.concept-elevation .device-label-bg {
+    fill: #000000;
+    fill-opacity: 0.45;
+    pointer-events: none;
+}
+
 .concept-elevation .device-label {
     font-size: 11px;
     pointer-events: none;
 }
 
+.concept-elevation .device-label.on-image {
+    text-shadow: 0 1px 2px rgba(0, 0, 0, 0.8);
+}
+
 .concept-elevation .opposite-hint {
     font-size: 9px;
     opacity: 0.8;