Add SVG/PNG/draw.io export for rack concepts, center the add-device hint

- Extract the elevation's <svg> body into inc/elevation_svg_content.html,
  shared by the on-page elevation and a new standalone export template
  so they never drift apart.
- Add "Export" dropdown above the elevation: Front/Rear as standalone
  SVG (self-contained inline styles, no dependency on the page's CSS,
  absolute device-type image URLs so the file still works once saved
  elsewhere), Front/Rear as PNG (rasterized entirely client-side via
  <canvas> from that same SVG - no new server-side image dependency),
  and the whole concept as a draw.io (.drawio / mxGraph XML) diagram
  with both faces side by side, reusing the identical block geometry.
- Center the "+ Add device" hint on empty units instead of
  right-aligning it.

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

netbox_rack_concept/elevation.py

@@ -116,6 +116,7 @@ class Elevation:
         self.rack_width = RACK_WIDTH
         self.rack_x = RACK_X
         self.rack_right = RACK_X + RACK_WIDTH
+        self.rack_center_x = RACK_X + RACK_WIDTH / 2
         self.width = RACK_X + RACK_WIDTH + MARGIN
         self.height = concept.u_height * UNIT_HEIGHT + MARGIN * 2