fix: handle plain HttpResponse from elevation SVG endpoint, unracked devices

- RackElevationOfflineSVGView called .render() unconditionally on the
  RackViewSet elevation action's response, but that action returns a
  plain HttpResponse (not a DRF Response) when render=svg is
  requested, causing AttributeError on every click.
- effective_width_positions() crashed with decimal.InvalidOperation
  when a device had no rack position set (e.g. surfaced via NetBox
  Topology Views, which includes unracked devices). Such devices now
  get the default full-width result instead of entering the Decimal
  conversion.

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

netbox_utilities/rack_width.py

@@ -122,6 +122,9 @@ def effective_width_positions(devices, placements=None):
     result = {}
 
     for device in devices:
+        if device.position is None:
+            result.setdefault(device.pk, (FULL_WIDTH, 1, "default"))
+            continue
         key = (
             device.rack_id,
             device.face,