v0.2.1: rework PNG export to draw directly on canvas, like netbox-topology-views

Investigated how MrBlake/mrb-netbox-topology-views does its PNG export:
it renders its topology natively onto a canvas (vis-network draws
there directly) and just calls canvas.toDataURL() - no SVG round-trip
at all. Applied the same principle here.

Previous approach loaded the standalone SVG export via <img src="blob:...">
and rasterized that onto a canvas - fragile, because the whole export
failed if that SVG document wasn't strictly valid XML (as our leaked
{# #} comment bug demonstrated) or if a single device type image
reference inside it couldn't be resolved.

New approach: a dedicated export/data/ endpoint returns the elevation's
plain pixel geometry as JSON (positions, sizes, colors, labels, image
URLs - the same numbers the SVG is built from), and export.js draws
that directly onto a <canvas> with native 2D primitives (fillRect,
strokeRect, fillText/strokeText, drawImage). Device type images are
fetched individually as same-origin blobs before drawing; one image
failing to load only drops that picture; the rest of the export still
succeeds. Added a combined front+rear PNG option.

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

netbox_rack_concept/__init__.py

@@ -1,6 +1,6 @@
 from netbox.plugins import PluginConfig
 
-__version__ = '0.2.0'
+__version__ = '0.2.1'
 
 
 class RackConceptConfig(PluginConfig):