feat: add offline SVG export for rack elevations

Adds a "SVG offline" download button on the rack detail page. It
reuses NetBox's own rack-elevation REST endpoint to render the SVG,
then strips embedded <a>/xlink:href links (which point at internal,
relative NetBox URLs and are useless once the file is saved) so the
result renders identically when opened standalone in any SVG viewer.

Gated by the new offline_svg_export_enabled plugin setting (on by
default).

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

netbox_utilities/templates/netbox_utilities/rack_offline_svg_buttons.html

@@ -0,0 +1,10 @@
+<a href="{% url 'plugins:netbox_utilities:rack_elevation_offline_svg' pk=rack.pk %}?face=front"
+   class="btn btn-outline-secondary" title="Vorderseite als linkfreie SVG-Datei herunterladen">
+  <i class="mdi mdi-download" aria-hidden="true"></i>
+  SVG offline (vorne)
+</a>
+<a href="{% url 'plugins:netbox_utilities:rack_elevation_offline_svg' pk=rack.pk %}?face=rear"
+   class="btn btn-outline-secondary" title="Rückseite als linkfreie SVG-Datei herunterladen">
+  <i class="mdi mdi-download" aria-hidden="true"></i>
+  SVG offline (hinten)
+</a>